Fair Value Disclosures [Abstract] Fair Value Measurements, Recurring and Nonrecurring Fair Value Measurements, Recurring and Nonrecurring [Table Text Block] Fair Value, by Balance Sheet Grouping Fair Value, by Balance Sheet Grouping [Table Text Block] Investments, Debt and Equity Securities [Abstract] SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate Schedule [Table] SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate Schedule [Table] Financial Asset, Period Past Due [Axis] Financial Asset, Period Past Due [Axis] Financial Asset, Period Past Due [Domain] Financial Asset, Period Past Due [Domain] Financing Receivables, Equal to Greater than 90 Days Past Due Financial Asset, Equal to or Greater than 90 Days Past Due [Member] Preferred equity investment SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items] Allowance for loan and lease losses Allowance for Loan and Lease Losses, Real Estate Number of investments marketed for sale Financing Receivable, Number Of investments Marketed For Sale Financing Receivable, Number Of investments Marketed For Sale Unpaid principal balance Impaired Financing Receivable, with Related Allowance, Unpaid Principal Balance Carrying value Mortgage Loans on Real Estate, Commercial and Consumer, Net Number of portfolio segments of financial receivables (segment) Financing Receivable, Portfolio Segments Number Represents the number of portfolio segments of financing receivables. Additional amount of financing receivables included in other assets Financing Receivable, Recorded in Other Assets Represents the amount of financing receivables recorded in other assets on the entity's balance sheet. Financing receivable, past due Financing Receivable, Past Due Financing receivable, nonaccrual status Financing Receivable, Nonaccrual Weighted average risk rating Debt And Equity Securities, Weighted Average Risk Rating Debt And Equity Securities, Weighted Average Risk Rating Stockholders' Equity Note [Abstract] Earnings Per Share Earnings Per Share [Abstract] Numerator Net Income (Loss) Attributable to Parent [Abstract] Basic Earnings: Net Income (Loss) Available to Common Stockholders, Basic [Abstract] Income attributable to SL Green common stockholders Net Income (Loss) Available to Common Stockholders, Basic Less: distributed earnings allocated to participating securities Distributed Earnings Net income attributable to SL Green common stockholders (numerator for basic earnings per share) Net Income (Loss) Available To Common Stockholders, After Distributed And Undistributed Earnings Allocation, Basic Net Income (Loss) Available To Common Stockholders, After Distributed And Undistributed Earnings Allocation, Basic Effect of Dilutive Securities: Dilutive Securities, Effect on Basic Earnings Per Share [Abstract] Add back: Dilutive effect of earnings allocated to participating securities Undistributed Earnings (Loss) Allocated to Participating Securities, Diluted Add back: Effect of dilutive securities (redemption of units to common shares) Dilutive Securities, Effect on Basic Earnings Per Share, Dilutive Convertible Securities Diluted Earnings: Net Income (Loss) Available to Common Stockholders, Diluted [Abstract] Income attributable to SL Green common stockholders (numerator for diluted earnings per share) Net Income (Loss) Available to Common Stockholders, Diluted Denominator Weighted Average Number of Shares Outstanding, Diluted [Abstract] Basic Shares: Weighted Average Number of Shares Outstanding, Basic [Abstract] Weighted average common stock outstanding (in shares) Weighted Average Number of Shares Outstanding, Basic Effect of Dilutive Securities: Incremental Weighted Average Shares Attributable to Dilutive Effect [Abstract] Operating Partnership units redeemable for common shares (in shares) Incremental Common Shares, Attributable to Redemption of Units to Common Shares Additional shares included in the calculation of diluted EPS as a result of the potentially dilutive effect of the redemption of units to common shares. Stock-based compensation plans (in shares) Incremental Common Shares Attributable to Dilutive Effect of Share-based Payment Arrangements Diluted weighted average common stock outstanding (in shares) Weighted Average Number of Shares Outstanding, Diluted Common stock shares excluded from the diluted shares outstanding (shares) Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount Equity [Abstract] Schedule of Stock by Class [Table] Schedule of Stock by Class [Table] Share Repurchase Program [Axis] Share Repurchase Program [Axis] Share Repurchase Program [Domain] Share Repurchase Program [Domain] 2016 Repurchase Program 2016 Repurchase Program [Member] 2016 Repurchase Program [Member] Class of Stock [Line Items] Class of Stock [Line Items] Stock repurchase program, authorized amount Stock Repurchase Program, Authorized Amount Number of increases to share repurchase program Share Repurchase Program, Number Of Increases To Share Repurchase Program Share Repurchase Program, Number Of Increases To Share Repurchase Program Stock repurchase program, authorized amount, total Stock Repurchase Program, Authorized Amount, Total Stock Repurchase Program, Authorized Amount, Total Number of shares purchased (in shares) Stock Repurchased During Period, Shares Maximum approximate dollar value of shares that may yet be purchased under the plan Treasury Stock Acquired, Average Cost Per Share Commitments and Contingencies Disclosure [Abstract] Commitments and Contingencies Commitments and Contingencies Disclosure [Text Block] SEC Schedule, 12-09, Valuation and Qualifying Accounts [Abstract] Schedule II - Valuation and Qualifying Accounts SEC Schedule, 12-09, Schedule of Valuation and Qualifying Accounts Disclosure [Text Block] Debt Disclosure [Abstract] Corporate Indebtedness Debt Disclosure [Text Block] Long-term Debt, Type [Axis] Long-term Debt, Type [Axis] Long-term Debt, Type [Domain] Long-term Debt, Type [Domain] Total fixed rate Fixed Rate [Member] Fixed Rate [Member] Total floating rate Debt Investment with Floating Interest Rate [Member] Represents the debt investment in mortgage loans having a floating interest rate. SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate, Loan Type [Axis] SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate, Loan Type [Axis] SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate, Loan Type [Domain] SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate, Loan Type [Domain] Mezzanine Loan with an Initial Maturity Date of March 2020 Mezzanine Loan with an Initial Maturity Date of March 2020 [Member] Mezzanine Loan with an Initial Maturity Date of March 2020 [Member] Mezzanine Loan with an Initial Maturity Date of September 2021 Mezzanine Loan with an Initial Maturity Date of September 2021 [Member] Mezzanine Loan with an Initial Maturity Date of September 2021 [Member] Mezzanine Loan with an Initial Maturity Date of April 2022 Mezzanine Loan with an Initial Maturity Date of April 2022 [Member] Mezzanine Loan with an Initial Maturity Date of April 2022 [Member] Mezzanine Loan With An Initial Maturity Of August 2022 Mezzanine Loan With An Initial Maturity Of August 2022 [Member] Mezzanine Loan With An Initial Maturity Of August 2022 Mezzanine Loan With Maturity June 2023 Mezzanine Loan With Maturity June 2023 [Member] Mezzanine Loan With Maturity June 2023 [Member] Mezzanine Loan, with an Initial Maturity Date of November 2023 Mezzanine Loan, with an Initial Maturity Date of November 2023 [Member] Mezzanine Loan, with an Initial Maturity Date of November 2023 [Member] Mezzanine Loan With An Initial Maturity Date Of December 2023 Mezzanine Loan With An Initial Maturity Date Of December 2023 [Member] Mezzanine Loan With An Initial Maturity Date Of December 2023 [Member] Mezzanine Loan with an Initial Maturity Date of June 2024 Mezzanine Loan with an Initial Maturity Date of June 2024 [Member] Mezzanine Loan with an Initial Maturity Date of June 2024 [Member] Mezzanine Loan, with an Initial Maturity Date of January 2025 Mezzanine Loan, with an Initial Maturity Date of January 2025 [Member] Mezzanine Loan, with an Initial Maturity Date of January 2025 [Member] Mezzanine Loan with an Initial Maturity Date of June 2027 Mezzanine Loan with an Initial Maturity Date of June 2027 [Member] Mezzanine Loan with an Initial Maturity Date of June 2027 [Member] Mezzanine Loan Sold In 2019 Mezzanine Loan Sold In 2019 [Member] Mezzanine Loan Sold In 2019 [Member] Mezzanine Loan with an Initial Maturity of January 2020 Mezzanine Loan with an Initial Maturity of January 2020 [Member] Mezzanine Loan with an Initial Maturity of January 2020 [Member] Mortgage/Mezzanine Loan Due March 2020 Mortgage/Mezzanine Loan Due March 2020 [Member] Mortgage/Mezzanine Loan Due March 2020 [Member] Mortgage and Mezzanine Loan with an Initial Maturity Date of April 2020 Mortgage and Mezzanine Loan with an Initial Maturity Date of April 2020 [Member] Mortgage and Mezzanine Loan with an Initial Maturity Date of April 2020 [Member] Mezzanine Loan with an Initial Maturity of July 2020 Mezzanine Loan with an Initial Maturity of July 2020 [Member] Mezzanine Loan with an Initial Maturity of July 2020 [Member] Mortgage/Mezzanine Loan Due August 2020 Mortgage/Mezzanine Loan Due August 2020 [Member] Mortgage/Mezzanine Loan Due August 2020 [Member] Mezzanine Loan Due September 2020 Mezzanine Loan Due September 2020 [Member] Mezzanine Loan Due September 2020 [Member] Mortgage And Mezzanine Loan With An Initial Maturity Date Of October 2020 Mortgage And Mezzanine Loan With An Initial Maturity Date Of October 2020 [Member] Mortgage And Mezzanine Loan With An Initial Maturity Date Of October 2020 [Member] Mezzanine Loan Due December 2021 Mezzanine Loan Due December 2021 [Member] Mezzanine Loan Due December 2021 [Member] Mortgage/Mezzanine Loan with an Initial Maturity of December 2020 Mortgage/Mezzanine Loan with an Initial Maturity of December 2020 [Member] Mortgage/Mezzanine Loan with an Initial Maturity of December 2020 [Member] Mortgage/Mezzanine Loan With An Initial Maturity Due April 2021 Mortgage/Mezzanine Loan With An Initial Maturity Due April 2021 [Member] Mortgage/Mezzanine Loan With An Initial Maturity Due April 2021 [Member] Mezzanine Loan With An Initial Maturity Due April 2021 Mezzanine Loan With An Initial Maturity Due April 2021 [Member] Mezzanine Loan With An Initial Maturity Due April 2021 [Member] Jr Mortgage Participation/Mezzanine Loan with an Initial Maturity of July 2021 Jr Mortgage Participation/Mezzanine Loan with an Initial Maturity of July 2021 [Member] Jr Mortgage Participation/Mezzanine Loan with an Initial Maturity of July 2021 [Member] Mezzanine Loan With An Initial Maturity Due July 2021 Mezzanine Loan With An Initial Maturity Due July 2021 [Member] Mezzanine Loan With An Initial Maturity Due July 2021 [Member] Mezzanine Loan Due December 2022 Mezzanine Loan Due December 2022 [Member] Mezzanine Loan Due December 2022 [Member] Mezzanine Loan Due December 2029 Mezzanine Loan Due December 2029 [Member] Mezzanine Loan Due December 2029 [Member] Mezzanine Loan Repaid In 2019 Mezzanine Loan Repaid In 2019 [Member] Mezzanine Loan Repaid In 2019 [Member] Mezzanine Loan Repaid In 2019, 2 Mezzanine Loan Repaid In 2019, 2 [Member] Mezzanine Loan Repaid In 2019, 2 [Member] Mezzanine Loan Repaid In 2019, 3 Mezzanine Loan Repaid In 2019, 3 [Member] Mezzanine Loan Repaid In 2019, 3 [Member] Mezzanine Loan Repaid In 2019, 4 Mezzanine Loan Repaid In 2019, 4 [Member] Mezzanine Loan Repaid In 2019, 4 [Member] Mortgage And Mezzanine Loan Repaid In 2019 Mortgage And Mezzanine Loan Repaid In 2019 [Member] Mortgage And Mezzanine Loan Repaid In 2019 [Member] Mezzanine Loan Repaid In 2019, 5 Mezzanine Loan Repaid In 2019, 5 [Member] Mezzanine Loan Repaid In 2019, 5 [Member] Mortgage/Jr.Mortgage Participation Loan Repaid In 2019 Mortgage/Jr.Mortgage Participation Loan Repaid In 2019 [Member] Mortgage/Jr.Mortgage Participation Loan Repaid In 2019 [Member] Mortgage And Mezzanine Loan Repaid In 2019, 2 Mortgage And Mezzanine Loan Repaid In 2019, 2 [Member] Mortgage And Mezzanine Loan Repaid In 2019, 2 [Member] Mortgage And Mezzanine Loan Repaid In 2019, 3 Mortgage And Mezzanine Loan Repaid In 2019, 3 [Member] Mortgage And Mezzanine Loan Repaid In 2019, 3 [Member] Mezzanine Loan Repaid In 2019, 6 Mezzanine Loan Repaid In 2019, 6 [Member] Mezzanine Loan Repaid In 2019, 6 [Member] Mezzanine Loan Repaid In 2019, 7 Mezzanine Loan Repaid In 2019, 7 [Member] Mezzanine Loan Repaid In 2019, 7 [Member] Mezzanine Loan Repaid In 2019, 8 Mezzanine Loan Repaid In 2019, 8 [Member] Mezzanine Loan Repaid In 2019, 8 [Member] Mezzanine Loan Repaid In 2019, 9 Mezzanine Loan Repaid In 2019, 9 [Member] Mezzanine Loan Repaid In 2019, 9 [Member] Debt investment Debt Investments in Mortgage Loans [Member] Represents debt investments in mortgage loans on real estate. SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items] Mortgage loans on real estate, interest rate SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate, Interest Rate Debt Investments Held [Abstract] Debt Investments Held [Abstract] Debt Investments Held [Abstract] Future Funding Obligations Loans and Leases Receivable, Funding Obligation Loans and Leases Receivable, Funding Obligation Senior Financing Mortgage Loans on Real Estate, Senior Debt on Investment Amount as of the balance sheet date of debt which holds a position senior to the entity's investment in a mortgage loan on real estate. Carrying Value Loans and Leases Receivable, Gross Amount participated out Mortgage Loans on Real Estate Loan Value Represents the amount of loan participation on property mortgage. Stockholders Equity [Table] Stockholders Equity [Table] Disclosure of information pertaining to changes in stockholders' equity during the period. Consolidated Entities [Axis] Consolidated Entities [Axis] Consolidated Entities [Domain] Consolidated Entities [Domain] SL Green Operating Partnership Subsidiaries [Member] Stockholders' Equity Stockholders Equity [Line Items] -- None. No documentation exists for this element. -- Numerator Net income attributable to SLGOP common unitholders Net Income (Loss) Allocated to, Common Unitholders This element represents amounts attributable to ROP common unitholders. Less: distributed earnings allocated to participating securities Income attributable to SLGOP common unitholders Net Income (Loss) Allocated to, Common Unitholders, Including Antidilutive Effect Net Income (Loss) Allocated to, Common Unitholders, Including Antidilutive Effect Denominator Stock-based compensation plans (shares) Incremental Common Units Attributable to Share based Payment Arrangements Incremental Common Units Attributable to Share based Payment Arrangements Equity Method Investments and Joint Ventures [Abstract] Schedule of Equity Method Investments [Table] Schedule of Equity Method Investments [Table] Product and Service [Axis] Product and Service [Axis] Product and Service [Domain] Product and Service [Domain] Management Service, Base Management Service, Base [Member] Related Party [Axis] Related Party [Axis] Related Party [Domain] Related Party [Domain] Joint venture Corporate Joint Venture [Member] Investment in Unconsolidated Joint Ventures Schedule of Equity Method Investments [Line Items] Management fees, base revenue Revenue from Contract with Customer, Including Assessed Tax Assets Equity Method Investment, Summarized Financial Information, Assets [Abstract] Commercial real estate property, net Real Estate Investment Property, Net Cash and restricted cash Cash and Cash Equivalents, at Carrying Value Tenant and other receivables and deferred rents receivable Accounts and Financing Receivable, after Allowance for Credit Loss Debt and preferred equity investments Other assets Other Assets Total assets Equity Method Investment, Summarized Financial Information, Assets Liabilities and equity Equity Method Investment, Summarized Financial Information, Liabilities and Equity [Abstract] Mortgages and other loans payable, net Secured Debt Deferred revenue Deferred Revenue Lease liabilities Operating Lease, Liability Other liabilities Other Liabilities Equity Equity Method Investment Summarized Financial Information, Equity Total liabilities and equity Equity Method Investment, Summarized Financial Information, Liabilities and Equity Company's investments in unconsolidated joint ventures Equity Method Investments Investments difference between carrying amount and underlying equity Equity Method Investment, Difference Between Carrying Amount and Underlying Equity Combined statements of income for the unconsolidated joint ventures Equity Method Investment, Summarized Financial Information, Income Statement [Abstract] Total revenues Equity Method Investment, Summarized Financial Information, Revenue Operating expenses Costs and Expenses Real estate taxes Real Estate Tax Expense Operating lease, rent Operating Lease, Expense Interest expense, net of interest income Interest Expense Amortization of deferred financing costs Amortization of Debt Issuance Costs Transaction related costs Other Cost and Expense, Operating Depreciation and amortization Depreciation, Depletion and Amortization, Nonproduction Total expenses Total Costs and Expenses Including Nonoperating Income (Expense) This element represents the total of the costs related to real estate revenues, including management, leasing, and development services and income (expense) from ancillary business-related activities. Loss on early extinguishment of debt Income (Loss) from Equity Method Investments Gains Losses on Extinguishment of Debt Represents the entity's share of the gain or loss on extinguishment of debt which is included in the equity income from unconsolidated joint ventures. Net (loss) income before gain on sale Equity Method Investment, Summarized Financial Information, Net Income (Loss) Company's equity in net (loss) income from unconsolidated joint ventures Income (Loss) from Equity Method Investments Loan loss reserve activity Financing Receivable, Allowance for Credit Loss [Roll Forward] Balance at beginning of year Expensed Provision for Loan, Lease, and Other Losses Recoveries Financing Receivable, Allowance for Credit Loss, Recovery Charge-offs and reclassifications Financing Receivable Allowance for Credit Losses Write Offs and Reclassifications Represents the amount of reclassifications and direct write-downs of financing receivables charged against the allowance. Balance at end of year Class of Stock [Axis] Class of Stock [Axis] Class of Stock [Domain] Class of Stock [Domain] 2015 ATM Equity Offering Program 2015 ATM Equity Offering Program [Member] 2015 New At-the-Market Equity Offering Program [Member] Series I Preferred Stock Series I Preferred Stock [Member] Outstanding nonredeemable series I preferred stock or outstanding series I preferred stock. Classified within stockholders' equity if nonredeemable or redeemable solely at the option of the issuer. Classified within temporary equity if redemption is outside the control of the issuer. Aggregate value of the shares of common stock to be sold Common Stock, Value of Shares to be Issued Represents the aggregate value of the shares to be sold under the stock issuance program. Preferred stock, shares outstanding (in shares) Preferred Stock, Shares Outstanding Dividend rate preferred units (as a percent) Preferred Stock, Dividend Rate, Percentage Perpetual Preferred stock, liquidation preference (in dollars per share) Preferred Stock, Liquidation Preference Per Share Perpetual Preferred stock, annual dividends per share (in dollars per share) Preferred Stock, Dividend Rate, Per-Dollar-Amount Contributions of net proceeds from sale of preferred stock Proceeds from Issuance of Preferred Stock and Preference Stock Disposal Group Name [Axis] Disposal Group Name [Axis] Disposal Group Name [Domain] Disposal Group Name [Domain] East 21 Street 66 East 21 Street 66 [Member] Represents the information pertaining to 21 East 66th Street, a joint venture property of the entity. Fifth Avenue, 521 Fifth Avenue, 521 [Member] Represents the information pertaining to 521 Fifth Avenue, a property owned by the entity. Spring Street 131 to 137 Spring Street 131 to 137 [Member] Represents information pertaining to property located at 131-137 Spring Street, owned by the entity. Stonehenge Portfolio Stonehenge Portfolio [Member] Stonehenge Portfolio [Member] 3 Columbus Circle Columbus Circle, 3 [Member] Represents the information pertaining to 3 Columbus Circle, a joint venture property of the entity. Mezzanine Loan Mezzanine Loan [Member] Mezzanine Loan [Member] 724 Fifth Avenue Fifth Avenue 724 [Member] Represents the information pertaining to 724 Fifth Avenue, a property acquired by the entity under the joint venture. Jericho Plaza Jericho Plaza [Member] Jericho Plaza [Member] 1745 Broadway Broadway, 1745 [Member] Represents the information pertaining to 1745 Broadway, a joint venture property of the entity. 175-225 Third Street Brooklyn, New York Third Street 175-225 [Member] Third Street 175-225 [Member] 1515 Broadway Broadway, 1515 [Member] Represents the information pertaining to 1515 Broadway, a property owned by the entity. 102 Greene Street Greene Street 102 [Member] Greene Street 102 [Member] 76 11th Avenue 76 11th Avenue [Member] 76 11th Avenue [Member] Madison Avenue 747 Madison Avenue 747 [Member] Represents the information pertaining to 747 Madison Avenue, a joint venture property of the entity. Investment, Name [Axis] Investment, Name [Axis] Investment, Name [Domain] Investment, Name [Domain] Participating Financing Due February 2022 Participating Financing Due February 2022 [Member] Participating Financing Due February 2022 [Member] Mezzanine Loans And Preferred Equity Mezzanine Loans And Preferred Equity [Member] Mezzanine Loans And Preferred Equity [Member] Schedule of Equity Method Investments [Line Items] Investments in unconsolidated joint ventures Acquisition, development and construction arrangements, carrying value Financing Receivable, before Allowance for Credit Loss Ownership Interest Sold Equity Method Investment, Ownership Percentage Gross asset valuation Disposal Group, Including Discontinued Operation, Consideration Gain (Loss) on Sale Equity Method Investment, Realized Gain (Loss) on Disposal Employee-related liabilities Employee-related Liabilities Number of notes owned Joint Venture, Number of Notes Owned Joint Venture, Number of Notes Owned (Loss) gain on sale of real estate, net Gains (Losses) on Sales of Investment Real Estate Noncontrolling Interest [Abstract] Noncontrolling Interests on the Company's Consolidated Financial Statements Noncontrolling Interest Disclosure [Text Block] Related Party Transactions [Abstract] Related Party Transactions Related Party Transactions Disclosure [Text Block] SEC Schedule, 12-09, Valuation and Qualifying Accounts Disclosure [Table] SEC Schedule, 12-09, Valuation and Qualifying Accounts Disclosure [Table] SEC Schedule, 12-09, Valuation Allowances and Reserves Type [Axis] SEC Schedule, 12-09, Valuation Allowances and Reserves Type [Axis] SEC Schedule, 12-09, Valuation Allowances and Reserves [Domain] SEC Schedule, 12-09, Valuation Allowances and Reserves [Domain] Deferred rent receivable—allowance Allowance for Deferred Rent Receivable [Member] Details pertaining to deferred rent receivable valuation allowance. Tenant and other receivables—allowance Allowance for Tenant and Other Receivables [Member] Details pertaining to tenant and other receivables valuation allowance. Changes in valuation allowance SEC Schedule, 12-09, Valuation and Qualifying Accounts Disclosure [Line Items] SEC Schedule, 12-09, Movement in Valuation Allowances and Reserves [Roll Forward] SEC Schedule, 12-09, Movement in Valuation Allowances and Reserves [Roll Forward] Balance at Beginning of Year SEC Schedule, 12-09, Valuation Allowances and Reserves, Amount Additions Charged Against Operations SEC Schedule, 12-09, Valuation Allowances and Reserves, Additions, Charge to Cost and Expense Uncollectible Accounts Written-off/Recovery (1) SEC Schedule, 12-09, Valuation Allowances and Reserves, Deduction Balance at End of Year Leases [Abstract] Fixed lease payments Operating Lease, Lease Income, Lease Payments Variable lease payments Operating Lease, Variable Lease Income Total lease payments Operating Lease Lease Income Fixed And Variable Lease Payments Operating Lease Lease Income Fixed And Variable Lease Payments Amortization of acquired above and below-market leases Amortization of above and below Market Leases Total rental revenue Operating Lease, Lease Income Statement of Stockholders' Equity [Abstract] Cash distribution declared, per common share (in dollars per share) Common Stock, Dividends, Per Share, Declared Organization, Consolidation and Presentation of Financial Statements [Abstract] Schedule of Ownership Interests [Table] Schedule of Ownership Interests [Table] Represents information related to ownership interests held by the entity. Ownership [Axis] Ownership [Axis] Ownership [Domain] Ownership [Domain] SLG Management LLC SLG Management LLC [Member] SLG Management LLC [Member] Legal Entity [Axis] Legal Entity [Axis] Entity [Domain] Entity [Domain] SL Green Management SLGreenManagement [Member] SLGreenManagement [Member] SL Green Operating Partnership Partnership Interest [Member] Variable Interest Entities [Axis] Variable Interest Entities [Axis] Variable Interest Entity, Classification [Domain] Variable Interest Entity, Classification [Domain] Service Corporation Variable Interest Entity, Primary Beneficiary [Member] Organization Ownership Interests [Line Items] -- None. No documentation exists for this element. -- Percentage of ownership in SL Green Management LLC owned by operating partnership (percent) Subsidiary of Limited Liability Company or Limited Partnership, Ownership Interest Noncontrolling interest in the operating partnership (as a percent) Noncontrolling Interest, Ownership Percentage by Noncontrolling Owners Class of Treasury Stock Class of Treasury Stock [Table Text Block] Schedule of Common Stock Issued and Proceeds Received Dividend Reinvestments Schedule of Common Stock Issued and Proceeds Received Dividend Reinvestments [Table Text Block] Schedule of Common Stock Issued and Proceeds Received Dividend Reinvestments [Table Text Block] Schedule of Earnings Per Share Schedule of Earnings Per Share, Basic and Diluted [Table Text Block] Accounting Policies [Abstract] Concentration Risk [Table] Concentration Risk [Table] Subsequent Event Type [Axis] Subsequent Event Type [Axis] Subsequent Event Type [Domain] Subsequent Event Type [Domain] Subsequent Event Subsequent Event [Member] Customer [Axis] Customer [Axis] Customer [Domain] Customer [Domain] Tenant 1 Tenant 1 [Member] Represents first tenant of the entity for which a concentration risk is disclosed. Credit Suisse Credit Suisse [Member] Credit Suisse [Member] Concentration Risk Benchmark [Axis] Concentration Risk Benchmark [Axis] Concentration Risk Benchmark [Domain] Concentration Risk Benchmark [Domain] Annualized rent Annualized Rent [Member] Annualized rent, when it serves as a benchmark in a concentration of risk calculation. Collective bargaining arrangements Workforce Subject to Collective Bargaining Arrangements [Member] Collective bargaining arrangements expiring in December 2019 Workforce Subject To Collective Bargaining Arrangements Expiring Within Two Years [Member] Workforce Subject To Collective Bargaining Arrangements Expiring Within Two Years [Member] Real Estate by Location [Axis] Real Estate by Location [Axis] Represents details pertaining to the locations of real estate properties of the entity. Real Estate by Location [Domain] Real Estate by Location [Domain] Identifies real estate properties of the entity by location. 11 Madison Avenue 11 Madison Ave [Member] 11 Madison Ave [Member] 1185 Avenue of the Americas Avenue of the Americas, 1185 [Member] Represents the information pertaining to 1185 Avenue of the Americas, a property owned by the entity. 420 Lexington Avenue Lexington Avenue, 420 [Member] Represents the information pertaining to 420 Lexington Avenue, a property owned by the entity. One Madison Ave 1 Madison Ave [Member] 1 Madison Ave [Member] East 220 Street 42 East 220 Street 42 [Member] Represents the information pertaining to 220 East 42nd Street, a property owned by the entity. Concentration Risk Type [Axis] Concentration Risk Type [Axis] Concentration Risk Type [Domain] Concentration Risk Type [Domain] Customer concentration Customer Concentration Risk [Member] Workforce concentration Labor Force Concentration Risk [Member] Concentration of Credit Risk Concentration Risk [Line Items] Number of tenants (tenants) Number of Tenants Represents the number of tenants to whom the facility is leased. Maximum percentage of annualized rent for any one tenant not individually disclosed (percent) (more than) Concentration Risk Percentage Threshold Represents the maximum percentage of concentration risk that is not individually disclosed. Percentage of concentration (percent) Concentration Risk, Percentage Number of collective bargaining agreements Number of Collective Bargaining Arrangements Represents the number of collective bargaining arrangements. Organization and Basis of Presentation Business Description and Basis of Presentation [Text Block] Dividends [Axis] Dividends [Axis] Dividends [Domain] Dividends [Domain] Ordinary Income Dividend Ordinary Income Dividend [Member] Ordinary Income Dividend [Member] Capital Gains Dividend Capital Gains Dividend [Member] Capital Gains Dividend [Member] Number of joint ventures with performance guarantees under master leases Number of Joint Ventures with Performance Guarantees under Master Leases Number of Joint Ventures with Performance Guarantees under Master Leases Deferred Lease Costs [Abstract] Deferred Lease Costs [Abstract] -- None. No documentation exists for this element. -- Portion of compensation capitalized Deferred Lease Costs Capitalized in Period Represents the amount of deferred lease costs capitalized during the period. Estimated average lease term (in years) Operating Leases, Estimated Average Lease Term Represents the estimated average term of the operating leases. Days past due for income recognition on debt and preferred equity investments to be suspended Financing Receivable Number of Days Past Due for Suspending Income Recognition The number of days by which payments become past due, at which income recognition on financing receivables is generally suspended. Income taxes Income Tax Disclosure [Abstract] Federal, state and local tax provision Income Tax Expense (Benefit) Dividends per share paid (in dollars per share) Common Stock, Dividends, Per Share, Cash Paid Statement of Cash Flows [Abstract] Statement [Table] Statement [Table] Preferred Units Preferred Stock [Member] Statement [Line Items] Statement [Line Items] Operating Activities Net Cash Provided by (Used in) Operating Activities [Abstract] Net income Net Income (Loss), Including Portion Attributable to Noncontrolling Interest Adjustments to reconcile net income to net cash provided by operating activities: Adjustments, Noncash Items, to Reconcile Net Income (Loss) to Cash Provided by (Used in) Operating Activities [Abstract] Depreciation and amortization Depreciation, Depletion and Amortization Equity in net loss (income) from unconsolidated joint ventures Distributions of cumulative earnings from unconsolidated joint ventures Proceeds from Equity Method Investment, Distribution Equity in net gain on sale of interest in unconsolidated joint venture interest/real estate Purchase price and other fair value adjustments Purchase Price, Fair Value Adjustment, Gain (Loss) Purchase Price, Fair Value Adjustment, Gain (Loss) Depreciable real estate reserves and impairment Depreciable Real Estate Reserves, Cash Flow Effect Depreciable Real Estate Reserves, Cash Flow Effect Loss (gain) on sale of real estate, net Loan loss reserves and other investment reserves, net of recoveries Gain on sale of investments in marketable securities Available-for-sale Securities, Gross Realized Gain (Loss) Loss on early extinguishment of debt Gain (Loss) on Extinguishment of Debt Deferred rents receivable Straight Line Rent Non-cash lease expense Non Cash Lease Expense Non Cash Lease Expense Other non-cash adjustments Other Noncash Income (Expense) Changes in operating assets and liabilities: Increase (Decrease) in Operating Capital [Abstract] Tenant and other receivables Increase (Decrease) in Accounts Receivable Related party receivables Increase (Decrease) in Due from Related Parties, Current Deferred lease costs Increase (Decrease) in Deferred Leasing Fees Other assets Increase (Decrease) in Other Operating Assets Accounts payable, accrued expenses, other liabilities and security deposits Increase (Decrease) in Accrued Liabilities and Other The change during the period in carrying value of accounts payable, accrued expenses and fair value of hedges, due within one year or operating cycle. Deferred revenue Increase (Decrease) in Deferred Liabilities Change in lease liability - operating leases Increase (Decrease) In Operating Lease Liability Increase (Decrease) In Operating Lease Liability Net cash provided by operating activities Net Cash Provided by (Used in) Operating Activities Investing Activities Net Cash Provided by (Used in) Investing Activities [Abstract] Acquisitions of real estate property Payments to Acquire Real Estate Additions to land, buildings and improvements Payments to Acquire Property, Plant, and Equipment Acquisition deposits and deferred purchase price Payments For Acquisition Deposits And Deferred Purchase Price Payments For Acquisition Deposits And Deferred Purchase Price Investments in unconsolidated joint ventures Payments to Acquire Equity Method Investments Distributions in excess of cumulative earnings from unconsolidated joint ventures Proceeds from Equity Method Investment, Distribution, Return of Capital Net proceeds from disposition of real estate/joint venture interest Proceeds from Real Estate and Real Estate Joint Ventures Proceeds from sale of marketable securities Proceeds from Sale and Maturity of Marketable Securities Other investments Payments For (Proceeds From) Other Investments Payments For (Proceeds) Other Investments Origination of debt and preferred equity investments Origination Of Debt And Preferred Equity Investments Origination Of Debt And Preferred Equity Investments Repayments or redemption of debt and preferred equity investments Repayments Or Redemption Of Debt And Preferred Equity Investments Repayments Or Redemption Of Debt And Preferred Equity Investments Net cash provided by investing activities Net Cash Provided by (Used in) Investing Activities Financing Activities Net Cash Provided by (Used in) Financing Activities [Abstract] Proceeds from mortgages and other loans payable Proceeds from Issuance of Secured Debt Repayments of mortgages and other loans payable Repayments of Secured Debt Proceeds from revolving credit facility and senior unsecured notes Proceeds from Issuance of Unsecured Debt Repayments of revolving credit facility and senior unsecured notes Repayments of Unsecured Debt Payment of debt extinguishment costs Payment for Debt Extinguishment or Debt Prepayment Cost Proceeds from stock options exercised and DRSPP issuance Proceeds from Stock Options Exercised Repurchase of common stock Payments for Repurchase of Common Stock Redemption of preferred stock Payments for Repurchase of Redeemable Preferred Stock Redemption of OP units Payments for Repurchase of Other Equity Distributions to noncontrolling interests in other partnerships Distributions to Noncontrolling Interests Other Partnerships The cash outflow for distributions to the entity's partner from a consolidated joint venture. Contributions from noncontrolling interests in other partnerships Proceeds from Noncontrolling Interests in Other Partnerships The cash inflow from the capital received in cash from a partner in a consolidated partnership during the period. Acquisition of subsidiary interest from noncontrolling interest Payments to Acquire Interest in Subsidiaries and Affiliates Distributions to noncontrolling interests in the Operating Partnership Distribution to Noncontrolling Interest Operating Partnership The cash outflow for distributions to partners in a partnership. Dividends/Distributions paid on common and preferred stock/units Payments of Dividends Other obligations related to loan participations Proceeds from Participating Mortgage Loan The cash inflow from participating mortgage loans and other mortgage obligations during the reporting period. Tax withholdings related to restricted share awards Payment, Tax Withholding, Share-based Payment Arrangement Deferred loan costs Payments of Financing Costs Net cash used in financing activities Net Cash Provided by (Used in) Financing Activities Net (decrease) increase in cash, cash equivalents, and restricted cash Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents, Period Increase (Decrease), Including Exchange Rate Effect Cash, cash equivalents, and restricted cash at beginning of year Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents Cash, cash equivalents, and restricted cash at end of period Supplemental Cash Flow Information [Abstract] Supplemental Cash Flow Information [Abstract] Interest paid Interest Paid, Excluding Capitalized Interest, Operating Activities Income taxes paid Income Taxes Paid Supplemental Disclosure of Non-Cash Investing and Financing Activities: Cash Flow, Noncash Investing and Financing Activities Disclosure [Abstract] Issuance of units in the operating partnership Partners' Capital Account, Sale of Units Conversion of units in the operating partnership Partners' Capital Account, Redemptions Redemption of units in the operating partnership for a joint venture sale Redemption Of Units In Operating Partnership For Sale Of Joint Venture Redemption Of Units In Operating Partnership For Sale Of Joint Venture Exchange of debt investment for real estate or equity in joint venture Transfer from Investments Issuance of preferred units relating to the real estate acquisition Stock Issued During Period, Value, Acquisitions Tenant improvements and capital expenditures payable Capital Expenditures Incurred but Not yet Paid Fair value adjustment to noncontrolling interest in the operating partnership Operating Partnership Noncontrolling Interest Fair Value Adjustment Represents the fair value adjustment to the noncontrolling interest in the operating partnership. Deconsolidation of a subsidiary Deconsolidation Real Estate Investments Represents the change in the carrying value of real estate investments resulting from deconsolidation. Transfer of assets to assets held for sale Transfer to Net Assets Held For Sale Value of assets transferred to assets held-for-sale in noncash transactions during the reporting period. Transfer of liabilities related to assets held for sale Transfer to Liabilities Related to Net Assets Held For Sale Value of liabilities transferred to liabilities related to assets held-for-sale in noncash transactions during the reporting period. Removal of fully depreciated commercial real estate properties Removal of Fully Depreciated Real Estate Assets Removal of Fully Depreciated Real Estate Assets Contribution to consolidated joint venture by noncontrolling interest Increase (Decrease) in Noncontrolling Interest in Joint Ventures Increase (Decrease) in Noncontrolling Interest in Joint Ventures Share repurchase payable Share Repurchases Payable Share Repurchases Payable Recognition of right of use assets and related lease liabilities Right-of-Use Asset Obtained in Exchange for Operating Lease Liability Cash and cash equivalents Restricted cash Restricted Cash and Cash Equivalents Total cash, cash equivalents, and restricted cash Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Abstract] Deferred leasing costs Deferred Costs, Leasing, Gross Less: accumulated amortization Deferred Costs, Leasing, Accumulated Amortization Deferred costs, net Deferred Costs, Leasing, Net Discontinued Operations and Disposal Groups [Abstract] Disposal Groups, Including Discontinued Operations [Table] Disposal Groups, Including Discontinued Operations [Table] Real Estate, Type of Property [Axis] Real Estate, Type of Property [Axis] Real Estate, Property Type [Domain] Real Estate [Domain] Development Rights Development Properties [Member] Represents the development properties owned by the entity. Suburban Suburban [Member] Represents information pertaining to property located in Suburban areas. 1640 Flatbush Avenue 1640 Flatbush Avenue [Member] 1640 Flatbush Avenue [Member] 562 Fifth Avenue 562 Fifth Avenue [Member] 562 Fifth Avenue [Member] 1010 Washington Boulevard Washington Boulevard 1010 [Member] Represents the information pertaining to 1010 Washington Boulevard, a property owned by the entity. Spring Street 115 Spring Street 115 [Member] Spring Street 115 [Member] 2 Herald Square 2 Herald Square [Member] 2 Herald Square [Member] 400 Summit Lake Drive Summit Lake Drive 400 [Member] Represents the information pertaining to 400 Summit Lake Drive, a property owned by the entity. Upper East Side Assemblage Upper East Side Assemblage [Member] Upper East Side Assemblage [Member] 1-6 International Drive 1-6 International Drive [Member] 1-6 International Drive [Member] 635 Madison Avenue Madison Avenue 635 [Member] Madison Avenue 635 [Member] 115-117 Stevens Avenue Stevens Avenue, 115 to 117 [Member] Represents the information pertaining to 115-117 Stevens Avenue, a property owned by the entity. 600 Lexington Avenue 600 Lexington Ave [Member] 600 Lexington Ave [Member] 125 Chubb Way Chubb Way 125 [Member] Represents the information pertaining to 125 Chubb Way, a property owned by the entity. 16 Court Street Court Street, 16 [Member] Represents the information pertaining to 16 Court Street, a joint venture property of the entity. 680-750 Washington Boulevard 680/750 Washington Boulevard [Member] 680/750 Washington Boulevard [Member] 520 White Plains Road White Plains Road, 520 [Member] Represents the information pertaining to 520 White Plains Road, a property owned by the entity. Property Dispositions and Assets Held for Sale Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] Unaudited Approximate Usable Square Feet Area of Real Estate Property Gain on Sale Discontinued Operation, Gain (Loss) on Disposal of Discontinued Operation, Net of Tax Purchase price and other fair value adjustments Ownership percentage in disposed asset Disposal Group Including Discontinued Operation, Ownership Percentage in Disposed Asset Disposal Group Including Discontinued Operation, Ownership Percentage in Disposed Asset Investment in joint venture, ownership percentage Investment in Joint Venture, Partners' Ownership Percentage Investment in Joint Venture, Partners' Ownership Percentage Deconsolidation of ownership interest percentage Disposal Group Including Discontinued Operation Deconsolidation Of Ownership Interest Percentage Disposal Group Including Discontinued Operation Deconsolidation Of Ownership Interest Percentage Impairment charge Disposal Group, Not Discontinued Operation, Loss (Gain) on Write-down Joint venture ownership percentage sold Joint Venture Ownership Percentage Sold Represents the ownership percentage of the joint venture sold to the incoming partner. Subsequent Event [Table] Subsequent Event [Table] 315 West 33rd Street 315 West 33rd Street [Member] 315 West 33rd Street [Member] Subsequent Event [Line Items] Subsequent Event [Line Items] Gross asset valuation of property to be sold Share-based Payment Arrangement [Abstract] Schedule of Share-based Compensation Arrangements by Share-based Payment Award [Table] Schedule of Share-based Compensation Arrangements by Share-based Payment Award [Table] Award Type [Axis] Award Type [Axis] Award Type [Domain] Award Type [Domain] Stock options Share-based Payment Arrangement, Option [Member] Restricted Stock Awards Restricted Stock [Member] Share-based Compensation Arrangement by Share-based Payment Award [Line Items] Share-based Compensation Arrangement by Share-based Payment Award [Line Items] Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract] Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract] Dividend yield (as a percent) Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Dividend Rate Expected life of option (in years) Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Term Risk-free interest rate (as a percent) Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Risk Free Interest Rate Expected stock price volatility (as a percent) Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Volatility Rate Options Outstanding Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward] Balance at beginning of year (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Number Granted (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Net of Forfeitures Exercised (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period Lapsed or cancelled (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Options, Forfeitures and Expirations in Period Balance at end of period (in shares) Options exercisable at end of period (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Number Weighted average fair value of options granted during the period Share Based Compensation Arrangement by Share Based Payment Award, Grants in Period Fair Value Represents the weighted average fair value of awards granted during the period. Weighted Average Exercise Price Share-based Compensation Arrangement by Share-based Payment Award, Options, Weighted Average Exercise Price [Roll Forward] Share-based Compensation Arrangement by Share-based Payment Award, Options, Weighted Average Exercise Price [Roll Forward] Balance at beginning of year (in dollars per share) Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price Granted (in dollars per share) Share-based Compensation Arrangements by Share-based Payment Award, Options, Grants in Period, Weighted Average Exercise Price Exercised (in dollars per share) Share-based Compensation Arrangements by Share-based Payment Award, Options, Exercises in Period, Weighted Average Exercise Price Lapsed or cancelled (in dollars per share) Share-based Compensation Arrangement by Share-based Payment Award, Options, Forfeitures and Expirations in Period, Weighted Average Exercise Price Balance at end of period (in dollars per share) Options exercisable at end of period (in dollars per share) Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Weighted Average Exercise Price Summary of restricted stock Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward] Balance at beginning of year (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number Granted (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period Cancelled (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Forfeited in Period Balance at end of period (in shares) Vested during the period (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period Compensation expense recorded Share-based Payment Arrangement, Expense Plan Name [Axis] Plan Name [Axis] Plan Name [Domain] Plan Name [Domain] Fourth Amended and Restated 2003 Stock Option and Incentive Plan Fourth Amended and Restated 2003 Stock Option and Incentive Plan [Member] Fourth Amended and Restated 2003 Stock Option and Incentive Plan [Member] Stock options, stock appreciation rights and other awards Stock Options Stock Appreciation Rights and Other Awards [Member] Represents the stock options, stock appreciation rights and other awards granted by the entity. Thes do not deliver full value and have an expiration date. Third Amendment and Restated 2005 Stock Option and Incentive Plan Third Amendment and Restated 2005 Stock Option and Incentive Plan [Member] Third Amendment and Restated 2005 Stock Option and Incentive Plan [Member] 2014 Outperformance Plan Long Term Outperformance Compensation Program 2014 [Member] Long Term Outperformance Compensation Program 2014 [Member] Deferred Stock Compensation Plan for Directors Deferred Stock Compensation Plan for Directors [Member] Represents the Deferred Stock Compensation Plan for Directors of the entity. All other awards Other Awards [Member] Represents awards granted by the entity that deliver the full value of the award upon vesting. Restricted Stock Awards Restricted Stock Awards [Member] Represents the restricted stock awards granted by the entity. Restricted stock are shares of stock for which sale is contractually or governmentally restricted for a given period of time. Class O LTIP Units Class O LTIP Units [Member] Class O LTIP Units [Member] LTIP units Long Term Incentive Plan Units [Member] LTIP units as awarded by a company to its employees as a form of incentive compensation. Performance Shares Performance Shares [Member] Employee Stock Employee Stock [Member] Statistical Measurement [Axis] Statistical Measurement [Axis] Statistical Measurement [Domain] Statistical Measurement [Domain] Minimum Minimum [Member] Maximum Maximum [Member] Vesting [Axis] Vesting [Axis] Vesting [Domain] Vesting [Domain] Vesting on August 31, 2017 Share-based Payment Arrangement, Tranche Three [Member] Vesting on August 31, 2018 Share-based Compensation Award, Tranche Four [Member] Share-based Compensation Award, Tranche Four [Member] Maximum fungible units that may be granted (in shares) Share Based Compensation Arrangement by Share Based Payment Award, Number of Fungible Units Authorized Represents the maximum number of fungible units authorized to be granted by the entity. Fungible units per share (in fungible units per share) Share Based Compensation Arrangement by Share Based Payment Award, Fungible Units Per Share Share Based Compensation Arrangement by Share Based Payment Award, Fungible Units Per Share Shares that may be issued if equal to fungible units (shares) (less than) Share Based Compensation Arrangement by Share Based Payment Award, Number of Shares to be Issued if Equal to Fungible Units Represents the number of shares that would be issued under the plan if shares issued is equal to fungible units; actual shares issued may be more or less depending on the type of awards issued and the ratio of fungible units to shares under each award. Fungible units Share-based Compensation Arrangement By Share-based Payment Award, Number Of Fungible Units Available For Grant Share-based Compensation Arrangement By Share-based Payment Award, Number Of Fungible Units Available For Grant Award expiration period (in years) Share-based Compensation Arrangement by Share-based Payment Award, Expiration Period Options vesting period (in years) Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period Period of commencement of option vesting, from date of grant (in years) Share Based Compensation Arrangement by Share Based Payment Award, Period of Award Vesting Commencement Represents the period from the date of grant of the awards for the commencement of vesting of awards. Preferred unit distributions as a percentage of common unit distributions Share Based Compensation Arrangement by Share Based Payment Award, Preferred Unit Distributions as a Percentage of Common Unit Distributions Share Based Compensation Arrangement by Share Based Payment Award, Preferred Unit Distributions as a Percentage of Common Unit Distributions Remaining weighted average contractual life of the options outstanding (in years) Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Remaining Contractual Term Remaining weighted average contractual life of the options exercisable (in years) Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Weighted Average Remaining Contractual Term Compensation expense Total unrecognized compensation cost related to unvested stock awards Share-based Payment Arrangement, Nonvested Award, Cost Not yet Recognized, Amount Weighted average period for recognition of compensation cost related to unvested stock awards (in years) Share-based Payment Arrangement, Nonvested Award, Cost Not yet Recognized, Period for Recognition Annual award vesting rate, low end of range (as a percent) Share Based Compensation Arrangement by Share Based Payment Award, Award Vesting Rate, Low End of Range Represents the low end of the range of annual vesting rates that occurs once performance criteria are reached. Annual award vesting rate, high end of range (as a percent) Share Based Compensation Arrangement by Share Based Payment Award, Award Vesting Rate, High End of Range Represents the low end of the range of annual vesting rates that occurs once performance criteria are reached. Fair value of restricted stock vested during the period Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period, Fair Value Capitalized to assets associated with compensation expense related to our long-term compensation plans, restricted stock and stock options Share-based Payment Arrangement, Amount Capitalized Share-based Payment Arrangement, Noncash Expense Share-based Payment Arrangement, Noncash Expense Maximum award to be earned, if performance reaches maximum threshold, first performance period Share Based Compensation Arrangement By Share Based Payment Award, Maximum Award To Be Earned, If Performance Reaches Maximum Threshold, First Performance Period Share Based Compensation Arrangement By Share Based Payment Award, Maximum Award To Be Earned, If Performance Reaches Maximum Threshold, First Performance Period Maximum award to be earned, if performance reaches maximum threshold, first performance period Share Based Compensation Arrangement By Share Based Payment Award, Maximum Award To Be Earned, If Performance Reaches Maximum Threshold, Second Performance Period Share Based Compensation Arrangement By Share Based Payment Award, Maximum Award To Be Earned, If Performance Reaches Maximum Threshold, Second Performance Period Percentage of units vested Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Rights, Percentage Award period (in years) Share-based Compensation Arrangement by Share--based Payment Award, Stock Appreciation Period Share-based Compensation Arrangement by Share--based Payment Award, Stock Appreciation Period Maximum percentage of the annual retainer fee, chairman fees and meeting fees that may be deferred by non-employee directors (percent) Deferred Compensation Arrangement with Individual Maximum Percentage of the Compensation that May be Deferred Represents the maximum percentage of the compensation that may be deferred by employees of the entity under deferred compensation plans. Awards granted (in shares) Shares issued (in shares) Deferred Compensation Arrangement with Individual, Shares Issued Awards outstanding (in shares) Number of shares available for issuance (shares) Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized Shares of common stock available for issuance (shares) Share-based Compensation Arrangement by Share-based Payment Award, Purchase Price of Common Stock, Percent Duration of each offering period starting the first day of each calendar quarter (in months) Share Based Compensation Arrangement by Share Based Payment Award, Duration of Offering Periods Represents the duration of each offering period during which the shares are offered for purchase under the stock-based compensation plan of the entity. Shares of common stock issued (shares) Common Stock, Shares Held in Employee Trust, Shares Preferred Equity, April 2021 Preferred Equity, April 2021 [Member] Preferred Equity, April 2021 [Member] Preferred Equity Redeemed in May 2017 Preferred Equity June 2022 [Member] Preferred Equity June 2022 [Member] Preferred equity investments Preferred Equity Investments in Mortgage Loans [Member] Represents preferred equity investments in mortgage loans on real estate. Properties Held for Sale and Property Dispositions Disposal Groups, Including Discontinued Operations, Disclosure [Text Block] SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation Disclosure [Abstract] SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation, by Property [Table] SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation, by Property [Table] Real Estate [Domain] 711 Third Avenue Third Avenue, 711 [Member] Represents the information pertaining to 711 Third Avenue, a property owned by the entity. 555 W. 57th Street W 57 Street 555 [Member] Represents the information pertaining to 555 W. 57th Street, a property owned by the entity. 461 Fifth Avenue Fifth Avenue, 461 [Member] Represents the information pertaining to 461 Fifth Avenue, a property owned by the entity. 750 Third Avenue Third Avenue, 750 [Member] Represents the information pertaining to 750 Third Avenue, a property owned by the entity. 625 Madison Avenue Madison Avenue, 625 [Member] Represents the information pertaining to 625 Madison Avenue, a property owned by the entity. 485 Lexington Avenue Lexington Avenue, 485 [Member] Represents the information pertaining to 485 Lexington Avenue, a property owned by the entity. 609 Fifth Avenue Fifth Avenue, 609 [Member] Represents the information pertaining to 609 Fifth Avenue, a property owned by the entity. 810 Seventh Avenue Seventh Avenue, 810 [Member] Represents the information pertaining to 810 Seventh Avenue, a property owned by the entity. 1350 Avenue of the Americas Avenue of the Americas 1350 [Member] Represents information pertaining to 1350 Avenue of the Americas, a property owned by the entity. 1-6 Landmark Square Landmark Square 1 to 6 [Member] Represents the information pertaining to 1-6 Landmark Square, a property owned by the entity. 7 Landmark Square Landmark Square, 7 [Member] Represents the information pertaining to 7 Landmark Square, a property owned by the entity. 1055 Washington Boulevard Washington Boulevard 1055 [Member] Represents the information pertaining to 1055 Washington Boulevard, a property owned by the entity. 100 Church Street Church Street, 100 [Member] Represents the information pertaining to 100 Church Street, a property owned by the entity. 125 Park Avenue Park Avenue, 125 [Member] Represents the information pertaining to 125 Park Avenue, a property owned by the entity. Williamsburg Williamsburg [Member] Represents information pertaining to properties located at Williamsburg, owned by the entity. 110 East 42nd Street East 42nd Street 110 [Member] Represents the information pertaining to 110 East 42nd Street, a property partially owned by the entity. 400 East 58th Street East 400 Street 58 [Member] Represents information pertaining to 400 East 58th Street, a property owned by the entity. 762 Madison Avenue Madison Avenue 762 [Member] Represents information pertaining to property located at 762 Madison Avenue, owned by the entity. 304 Park Avenue Park Avenue 304 [Member] Represents information pertaining to property located at 304 Park Avenue, owned by the entity. 635 Sixth Avenue Sixth Avenue 635 [Member] Represents information pertaining to property located at 635 Sixth Avenue, owned by the entity. 641 Sixth Avenue Sixth Avenue 641 [Member] Represents information pertaining to property located at 641 Sixth Avenue, owned by the entity. 1080 Amsterdam Amsterdam 1080 [Member] Represents the information pertaining to 1080 Amsterdam, a property owned by the entity. 752-760 Madison Avenue 752-760 Madison Avenue [Member] 752-760 Madison Avenue [Member] 719 Seventh Avenue Seven One Nine Seventh Avenue [Member] Seven One Nine Seventh Avenue [Member] 110 Greene Street 110 Greene Street [Member] 110 Greene Street [Member] 185 Broadway 185 Broadway [Member] 185 Broadway [Member] 30 East 40th Street 30 East 40th Street [Member] 30 East 40th Street [Member] 113 Greene Street 113 Greene Street [Member] 113 Greene Street [Member] 712 Madison Avenue 712 Madison Avenue [Member] 712 Madison Avenue [Member] 106 Spring Street 106 Spring Street Member [Member] 106 Spring Street Member [Member] 410 Tenth Avenue 410 Tenth Avenue [Member] 410 Tenth Avenue [Member] Other Other Property [Member] SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation [Line Items] SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation [Line Items] Encumbrances SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation, Amount of Encumbrances Initial Cost SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation, Initial Cost [Abstract] Land SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation, Initial Cost of Land Building & Improvements SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation, Initial Cost of Building and Improvements Cost Capitalized Subsequent To Acquisition Real Estate and Accumulated Depreciation, Costs Capitalized Subsequent to Acquisition [Abstract] -- None. No documentation exists for this element. -- Land Real Estate and Accumulated Depreciation, Costs Capitalized Subsequent to Acquisition of Land Represents the carrying amount as of the balance sheet date of land costs that were capitalized after acquisition. Building & Improvements Real Estate and Accumulated Depreciation, Costs Capitalized Subsequent to Acquisition of Building and Improvements Represents the carrying amount as of the balance sheet date of building and improvement costs that were capitalized after acquisition. Gross Amount at Which Carried at Close of Period SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate, Gross [Abstract] Land SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation, Land, Amount Building & Improvements SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation, Building and Improvements, Amount Total SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate, Gross Accumulated Depreciation SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate, Accumulated Depreciation Interest in property (as a percent) Noncontrolling Interest, Ownership Percentage by Parent Mortgages and Other Loans Payable -- None. No documentation exists for this element. -- Mortgages and Other Loans Payable Stockholders' Equity of the Company Stockholders' Equity Note Disclosure [Text Block] Schedule of Accumulated Other Comprehensive Income Schedule of Accumulated Other Comprehensive Income (Loss) [Table Text Block] Real Estate Property Ownership [Axis] Real Estate Property Ownership [Axis] Real Estate Properties [Domain] Real Estate Properties [Domain] Consolidated properties Consolidated Properties [Member] Unconsolidated Properties Unconsolidated Properties [Member] 2019 Operating Leases, Future Minimum Payments Receivable, Current 2020 Operating Leases, Future Minimum Payments Receivable, in Two Years 2021 Operating Leases, Future Minimum Payments Receivable, in Three Years 2022 Operating Leases, Future Minimum Payments Receivable, in Four Years 2023 Operating Leases, Future Minimum Payments Receivable, in Five Years Thereafter Operating Leases, Future Minimum Payments Receivable, Thereafter Total future minimum payments receivable Operating Leases, Future Minimum Payments Receivable Schedule III - Real Estate and Accumulated Depreciation SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation Disclosure [Text Block] Segment Reporting [Abstract] Schedule of selected results of operations and selected asset information Schedule of Segment Reporting Information, by Segment [Table Text Block] Equity Components [Axis] Equity Components [Axis] Equity Component [Domain] Equity Component [Domain] Common Stock Common Stock [Member] Series I Preferred Units Units, outstanding Shares, Outstanding Common units redemption, period of restriction Common Units Redemption, Period Of Restriction Common Units Redemption, Period Of Restriction Conversion of stock, shares issued (in shares) Conversion of Stock, Shares Issued Number of units of operating partnership owned by the noncontrolling interest unit holders (units) Minority Interest Units of Partnership Owned by Noncontrolling Owners Represents the number of partnership units owned by the noncontrolling unit holders. Fair Value, Recurring and Nonrecurring [Table] Fair Value, Recurring and Nonrecurring [Table] Balance Sheet Location [Axis] Balance Sheet Location [Axis] Balance Sheet Location [Domain] Balance Sheet Location [Domain] Accrued Interest Payable and Other Accrued Interest Payable And Other [Member] Accrued Interest Payable And Other [Member] 919 Third Avenue Third Avenue, 919 [Member] Represents the information pertaining to 919 Third Avenue, a property owned by the entity. Geographical [Axis] Geographical [Axis] Geographical [Domain] Geographical [Domain] Business Acquisition [Axis] Business Acquisition [Axis] Business Acquisition, Acquiree [Domain] Business Acquisition, Acquiree [Domain] Total Fixed Rate Mortgage and Jr Mortgage Loan with an Initial Maturity Date of April 2017 Mortgage And Jr Mortgage Loan With An Initial Maturity Date Of April 2017 [Member] Mortgage And Jr Mortgage Loan With An Initial Maturity Date Of April 2017 [Member] Financial Instrument [Axis] Financial Instrument [Axis] Financial Instruments [Domain] Financial Instruments [Domain] Equity marketable securities Equity Securities [Member] Mortgage-backed securities Commercial Mortgage Backed Securities [Member] Fair Value Hierarchy and NAV [Axis] Fair Value Hierarchy and NAV [Axis] Fair Value, Measurements, Fair Value Hierarchy [Domain] Fair Value Hierarchy and NAV [Domain] Level 1 Fair Value, Inputs, Level 1 [Member] Level 2 Fair Value, Inputs, Level 2 [Member] Level 3 Fair Value, Inputs, Level 3 [Member] Measurement Basis [Axis] Measurement Basis [Axis] Fair Value Measurement [Domain] Fair Value Measurement [Domain] Portion at Fair Value Measurement Portion at Fair Value Measurement [Member] Carrying Value Reported Value Measurement [Member] Fair Value Estimate of Fair Value Measurement [Member] Fair Value of Financial Instruments Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] Marketable securities Debt Securities, Available-for-sale Interest rate cap and swap agreements (included in other assets) Derivative Asset, Fair Value, Gross Asset Interest rate cap and swap agreements (included in accrued interest payable and other liabilities) Derivative Liability, Fair Value, Gross Liability Asset impairment loss Asset Impairment Charges Notes receivable SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate, Face Amount of Mortgages Interest receivable Interest Receivable Purchase price fair value adjustment Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Consideration Transferred Purchase price and other fair value adjustment Purchase Price Fair Value Adjustment Gross The impact of fair value adjustments on purchase price and it's impact on net income or loss for the period, gross. Fixed rate debt Long-term Debt, Percentage Bearing Fixed Interest, Amount Variable rate debt Long-term Debt, Percentage Bearing Variable Interest, Amount Total Long-term Debt, Fair Value Estimated fair value of debt and preferred equity investments, low end of range Mortgage Loans on Real Estate, Fair Value, Low End of Range Represents the low end of the range of estimated fair value for investments in mortgage loans on real estate. Estimated fair value of debt and preferred equity investments, high end of range Mortgage Loans on Real Estate Fair Value, High End of Range Represents the high end of the range of estimated fair value for investments in mortgage loans on real estate. Statement of Partners' Capital [Abstract] Adjustments for New Accounting Pronouncements [Axis] Adjustments for New Accounting Pronouncements [Axis] Type of Adoption [Domain] Type of Adoption [Domain] Accounting Standards Update 2014-09 Accounting Standards Update 2014-09 [Member] Accumulated Other Comprehensive Income (Loss) AOCI Attributable to Parent [Member] Noncontrolling Interests Noncontrolling Interest [Member] Partner Capital Components [Axis] Partner Capital Components [Axis] Partner Capital Components [Domain] Partner Capital Components [Domain] Partners' Interest General Partner [Member] Increase (Decrease) in Partner's Capital Increase (Decrease) in Stockholders' Equity [Roll Forward] Beginning Balance Partners' Capital, Including Portion Attributable to Noncontrolling Interest Beginning Balance (units) Partners' Capital Account, Units Adjusted Balance Partners' Capital, Adjusted Balance Net income Profit Loss, Less Preferred Distributions The consolidated profit or loss for the period, net of income taxes, including the portion attributable to the noncontrolling interest, less preferred distributions during the period. Other comprehensive income (loss) Other Comprehensive Income (Loss), Net of Tax, Less Portion Attributable to Noncontrolling Interest Other Comprehensive Income (Loss), Net of Tax, Less Portion Attributable to Noncontrolling Interest Preferred dividends Distributions Preferred Units Distributions declared for preferred units during the period. DRSPP proceeds (in units) Stock Issued During Period, Shares, Dividend Reinvestment Plan DRSPP proceeds Stock Issued During Period, Value, Dividend Reinvestment Plan Reallocation of noncontrolling interests in the operating partnership Reallocation of Noncontrolling Interest to (from) Operating Partnership Reallocation of Noncontrolling Interest to (from) Operating Partnership Equity component of repurchased exchangeable senior notes Adjustments to Additional Paid in Capital, Equity Component of Repurchased Senior Notes Adjustments to Additional Paid in Capital, Equity Component of Repurchased Senior Notes Deferred compensation plan and stock awards, net of forfeitures and tax withholdings (in units) Deferred Compensation Plan and Stock Award, Shares Number of shares issued during the period from a Deferred Compensation Plan and Stock Award. Deferred compensation plan and stock awards, net of forfeitures and tax withholdings Deferred Compensation Plan and Stock Award, Net Value of stock issued during the period from a deferred compensation plan and stock award, net. Repurchases of common units (in units) Treasury Stock, Shares, Acquired Repurchases of common units Treasury Stock, Value, Acquired, Cost Method Proceeds from stock options exercised (in units) Common Stock Issued During Period, Shares New Issues This element represents the number of new common stock issued during the period. Proceeds from stock options exercised Partners' Capital Account, Option Exercise Contributions to consolidated joint venture interests Payments to Acquire Interest in Joint Venture Deconsolidation of partially owned entity Noncontrolling Interest, Decrease from Deconsolidation Cash distributions to noncontrolling interests Noncontrolling Interest, Decrease from Distributions to Noncontrolling Interest Holders Cash distributions declared (none of which represented a return of capital for federal income tax purposes) Partners' Capital Account, Distributions Cumulative adjustment upon adoption of ASC 610-20 Cumulative Effect of New Accounting Principle in Period of Adoption Conversion of common units (in units) Partners' Capital Account, Units, Sale of Units Conversion of common units Stock Issued During Period, Value, Conversion of Units Redemption of common units Noncontrolling Interest, Decrease from Redemptions or Purchase of Interests Proceeds from stock options exercised (in shares) Proceeds from stock options exercised Stock Issued During Period, Value, Stock Options Exercised Ending Balance Ending Balance (units) Counterparty Name [Axis] Counterparty Name [Axis] Counterparty Name [Domain] Counterparty Name [Domain] Allianz Real Estate Allianz Real Estate [Member] Allianz Real Estate [Member] Quarterly distributions declared, per share/unit (usd per share/unit) Common Stock, Dividends, Per Share, Quarterly Distributions Common Stock, Dividends, Per Share, Quarterly Distributions Common Stock Authorized capital stock (in shares) Total Shares Authorized The maximum number of total capital shares permitted to be issued by an entity's charter and bylaws. Authorized shares, par value (in dollars per share) Total Par or Stated Value Per Share Face amount or stated value of capital stock per share; generally not indicative of the fair market value per share. Common stock, shares authorized (in shares) Common Stock, Shares Authorized Common stock, par value (in dollars per share) Common Stock, Par or Stated Value Per Share Excess stock, shares authorized (shares) Excess Stock, Shares Authorized Excess stock, par value (in dollars per share) Excess Stock, Par or Stated Value Per Share Face amount or stated value of excess stock per share, which is generally not indicative of the fair market value per share. Preferred stock, shares authorized (shares) Preferred Stock, Shares Authorized Preferred stock, par (in dollars per share) Preferred Stock, Par or Stated Value Per Share Shares, outstanding (in shares) Excess shares issued (shares) Excess Stock, Shares Issued Business Combinations [Abstract] Property Acquisitions Business Combination Disclosure [Text Block] Significant Accounting Policies Significant Accounting Policies [Text Block] Statement of Financial Position [Abstract] Consolidated VIEs Tenant and other receivables, allowance Allowance for Doubtful Accounts, Premiums and Other Receivables Deferred rents receivable, allowance Allowance for Deferred Rents Receivable The valuation allowances attributable to, for instance, credit risk associated with a lessee as of the balance sheet date to reduce the gross amount of deferred rents receivable, which would be presented in parentheses on the face of the balance sheet. Debt and preferred equity investments, net of discounts and deferred origination fees Unamortized Loan Commitment and Origination Fees and Unamortized Discounts or Premiums Preferred stock, par (in dollars per share) Preferred stock, liquidation preference (in dollars per share) Preferred stock, shares issued (in shares) Preferred Stock, Shares Issued Limited partner interests in Operating Partnership, limited partner common units outstanding (shares) Limited Partners' Capital Account, Units Outstanding Preferred units, liquidation preference (in dollars per share) Temporary Equity, Liquidation Preference Per Share Preferred units, shares issued (in shares) Preferred Units, Issued Preferred units, shares outstanding (in shares) Preferred Units, Outstanding Common stock, shares issued (in shares) Common Stock, Shares, Issued Common stock, shares outstanding (in shares) Common Stock, Shares, Outstanding Treasury stock, shares (in shares) Treasury Stock, Shares SL Green partner's capital, general partner common units outstanding (shares) General Partners' Capital Account, Units Outstanding SL Green partners' capital, limited partner common units outstanding (shares) Limited Partnership Units Held by General Partner This element represents Limited Partnership Units Held by General Partner. Land Land Building and improvements Investment Building and Building Improvements Building leasehold and improvements Building Leasehold Improvements, Gross Carrying amount at the balance sheet date of long-lived, depreciable asset that is an addition or improvement to assets held under lease arrangement as well as the initial investment in a leasehold interest. Right of use asset - financing leases Finance Lease, Right-of-Use Asset Accumulated depreciation Real Estate Investment Property, Accumulated Depreciation Real estate debt Accrued interest payable Interest Payable Lease liabilities Finance Lease, Liability Schedule Of Ground Lease Arrangements Schedule Of Ground Lease Arrangements [Table Text Block] Schedule Of Ground Lease Arrangements [Table Text Block] Finance lease, liability, maturity Finance Lease, Liability, Maturity [Table Text Block] Schedule of future minimum rental payments for opeating leases Lessee, Operating Lease, Liability, Maturity [Table Text Block] Quarterly Financial Information Disclosure [Abstract] Condensed Income Statement [Table] Condensed Income Statement [Table] Condensed Income Statements, Captions [Line Items] Condensed Income Statements, Captions [Line Items] Schedule of Quarterly Financial Information Quarterly Financial Information [Table Text Block] Investments in Unconsolidated Joint Ventures Equity Method Investments and Joint Ventures Disclosure [Text Block] Principles of Consolidation Consolidation, Policy [Policy Text Block] Investment in Commercial Real Estate Properties Investment in Real Estate [Policy Text Block] Disclosure of accounting policy for investments in real estate properties. Cash and Cash Equivalents Cash and Cash Equivalents, Policy [Policy Text Block] Restricted Cash Cash and Cash Equivalents, Restricted Cash and Cash Equivalents, Policy [Policy Text Block] Fair Value Measurements Fair Value of Financial Instruments, Policy [Policy Text Block] Investment in Marketable Securities Marketable Securities, Policy [Policy Text Block] Investments in Unconsolidated Joint Ventures Equity Method Investments [Policy Text Block] Deferred Lease Costs Deferred Lease Costs [Policy Text Block] Disclosure of accounting policy for deferral and amortization of deferred lease costs. Deferred Financing Costs Deferred Financing Costs [Policy Text Block] Disclosure of accounting policy for deferral and amortization of deferred financing costs. Revenue Recognition Revenue [Policy Text Block] Allowance for Loan Loss and Other Investment Reserves Loans and Leases Receivable, Allowance for Loan Losses Policy [Policy Text Block] Rent Expense Rent Expense [Policy Text Block] Disclosure of accounting policy for recognizing rent expense. Underwriting Commissions and Costs Underwriting Commissions and Costs, Policy [Policy Text Block] Underwriting Commissions and Costs, Policy [Policy Text Block] Exchangeable Debt Instruments Convertible Debt Instruments, Policy [Policy Text Block] Convertible Debt Instruments, Policy [Policy Text Block] Transaction cost Transaction Costs [Policy Text Block] Transaction Costs [Policy Text Block] Income Taxes Income Tax, Policy [Policy Text Block] Stock-Based Employee Compensation Plans Share-based Payment Arrangement [Policy Text Block] Derivative Instruments Derivatives, Policy [Policy Text Block] Earnings per Share of the Company Earnings Per Share, Policy [Policy Text Block] Earnings per Unit of the Operating Partnership Earnings Per Unit, Policy [Policy Text Block] Earnings Per Unit, Policy [Policy Text Block] Use of Estimates Use of Estimates, Policy [Policy Text Block] Concentrations of Credit Risk Concentration Risk, Credit Risk, Policy [Policy Text Block] Reclassification Reclassification, Policy [Policy Text Block] Accounting Standards Updates New Accounting Pronouncements, Policy [Policy Text Block] Preferred Stock Additional Paid- In-Capital Additional Paid-in Capital [Member] Treasury Stock Treasury Stock [Member] Retained Earnings Retained Earnings [Member] Increase (Decrease) in Stockholders' Equity Beginning Balance Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest Beginning Balance (in shares) Adjusted Balance Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest, Adjusted Balance Net income (loss) Profit (Loss) Less Minority Interest in Net Income (Loss) Operating Partnerships Net income including minority interest in partnerships. Other comprehensive loss Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent Preferred dividends Dividends, Preferred Stock DRSPP (in shares) Conversion of units of the Operating Partnership to common stock (in shares) Stock Issued During Period, Shares, Conversion of Units Conversion of units in the Operating Partnership to common stock Reallocation of noncontrolling interest in the Operating Partnership Deferred compensation plan and stock awards, net of forfeitures and tax withholdings (in shares) Repurchases of common stock (in shares) Contributions to consolidated joint venture interests Consolidation of Joint Venture Interest This element represent the net income (loss) in consolidation of joint venture interest. Cash distributions declared ( per common share, none of which represented a return of capital for federal income tax purposes) Dividends, Common Stock Deconsolidation of partially owned entity Ending Balance Ending Balance (in shares) Income Statement [Abstract] Revenues Revenues [Abstract] Rental revenue, net Operating Leases, Income Statement, Minimum Lease Revenue Investment income Investment Income, Interest and Dividend Other income Other Income Total revenues Revenues Expenses Costs and Expenses [Abstract] Operating expenses, including $18,106 in 2019, $17,823 in 2018, $21,400 in 2017 of related party expenses Operating Costs and Expenses Operating lease rent Operating Lease, Cost Interest expense, net of interest income Interest Expense, Other Loan loss and other investment reserves, net of recoveries Provision for Loan and Lease Losses Marketing, general and administrative Selling, General and Administrative Expense Equity in net (loss) income from unconsolidated joint ventures Equity in net gain on sale of interest in unconsolidated joint venture/real estate (Loss) gain on sale of real estate, net Gain (Loss) on Sale of Properties, Net of Applicable Income Taxes Depreciable real estate reserves and impairment Depreciable Real Estate Reserves, Effect on Earnings Depreciable Real Estate Reserves, Effect on Earnings Gain on sale of investment in marketable securities Loss on early extinguishment of debt Net income Net (income) loss attributable to noncontrolling interests: Net Income (Loss) Attributable to Noncontrolling Interest [Abstract] Noncontrolling interests in the Operating Partnership Noncontrolling Interest in Net Income (Loss) Operating Partnerships, Redeemable Net loss attributable to noncontrolling interests in other partnerships Noncontrolling Interest in Net Income (Loss) Other Noncontrolling Interests, Redeemable Preferred unit distributions Noncontrolling Interest in Net Income (Loss) Preferred Unit Holders, Redeemable Net income (loss) attributable to SL Green/SLGOP Net Income (Loss) Attributable to Parent Perpetual preferred stock dividends Net income attributable to SL Green common stockholders Basic earnings per share (in dollars per share) Earnings Per Share, Basic Basic earnings per unit (in dollars per share) Net Income (Loss), Per Outstanding Limited Partnership Unit, Basic, Net of Tax Diluted earnings per share (in dollars per share) Earnings Per Share, Diluted Diluted earnings per unit (in dollars per share) Net Income (Loss), Net of Tax, Per Outstanding Limited Partnership Unit, Diluted Basic weighted average common shares outstanding (in shares) Basic weighted average common units outstanding (in shares) Weighted Average Limited Partnership Units Outstanding, Basic Diluted weighted average common shares and common share equivalents outstanding (in shares) Diluted weighted average common units and common unit equivalents outstanding (in shares) Weighted Average Limited Partnership Units Outstanding, Diluted Operating expenses, paid to related parties Related Party Transaction, Expenses from Transactions with Related Party Schedule of Long-term Debt Instruments [Table] Schedule of Long-term Debt Instruments [Table] Credit Facility [Axis] Credit Facility [Axis] Credit Facility [Domain] Credit Facility [Domain] Revolving credit facility Revolving Credit Facility [Member] Line of Credit Line of Credit [Member] Term loan Term Loan [Member] Represents the term loan of the reporting entity. Debt Instrument [Axis] Debt Instrument [Axis] Debt Instrument, Name [Domain] Debt Instrument, Name [Domain] Revolving Credit Facility, Maturity March 31, 2022 Revolving Credit Facility, Maturity March 31, 2022 [Member] Revolving Credit Facility, Maturity March 31, 2022 [Member] Term Loan A, Maturity March 31, 2023 Term Loan A, Maturity March 31, 2023 [Member] Term Loan A [Member] Term Loan B, Maturity November 21, 2024 Term Loan B, Maturity November 21, 2024 [Member] Term Loan B [Member] 2012 Credit Facility Credit Facility 2017 [Member] Credit Facility 2017 [Member] Variable Rate [Axis] Variable Rate [Axis] Variable Rate [Domain] Variable Rate [Domain] LIBOR London Interbank Offered Rate (LIBOR) [Member] Corporate Indebtedness Debt Instrument [Line Items] Credit facility, maximum borrowing capacity Line of Credit Facility, Maximum Borrowing Capacity Number of extensions Line Of Credit Facility, Number Of Extensions Line Of Credit Facility, Number Of Extensions Term of extension Line Of Credit Facility, Term Of Extension Line Of Credit Facility, Term Of Extension Maximum borrowing capacity, optional expansion Line of Credit Facility, Optional Expansion, Maximum Borrowing Capacity The expanded maximum borrowing capacity available, which is subject to agreement with the entity. Interest rate added to base rate (as a percent) Debt Instrument, Basis Spread on Variable Rate Facility fee (as a percent) Line of Credit Facility, Commitment Fee Percentage Letters of credit Letters of Credit Outstanding, Amount Outstanding under line of credit facility Long-term Line of Credit Ability to borrow under line of credit facility Line of Credit Facility, Remaining Borrowing Capacity Long-term debt Long-term Debt Retirement Benefits [Abstract] Benefit Plans Pension and Other Postretirement Benefits Disclosure [Text Block] Schedule of Noncontrolling Interest Redeemable Noncontrolling Interest [Table Text Block] Schedule of Preferred Unit Activity Schedule of Preferred Units [Table Text Block] Schedule of Calculation of Numerator and Denominator in Earnings Per Unit Schedule of Calculation of Numerator and Denominator in Earnings Per Unit [Table Text Block] Schedule of Calculation of Numerator and Denominator in Earnings Per Unit [Table Text Block] Schedule of Multiemployer Plans [Table] Schedule of Multiemployer Plans [Table] Multiemployer Plan Type [Axis] Multiemployer Plan Type [Axis] Multiemployer Plans Type [Domain] Multiemployer Plans Type [Domain] Pension Plan Multiemployer Plans, Pension [Member] Multiemployer Plan Name [Axis] Multiemployer Plan Name [Axis] Multiemployer Plans Name [Domain] Multiemployer Plan Name [Domain] Health Plan Health Plan [Member] Represents the health plan that defines the amount of health benefits to be provided, usually as a function of one or more factors such as age, years of service or compensation. Other plans Multiemployer Plans Other Plans [Member] Represents all other plans, not elsewhere specified in the taxonomy, to which two or more unrelated employers contribute where assets contributed by one participating employer may be used to provide benefits to employees of other participating employers. Multiemployer Plans [Line Items] Multiemployer Plans [Line Items] Multiemployer plans, surcharge Multiemployer Plans, Surcharge [Fixed List] Plan contributions Multiemployer Plans, Plan Contributions Multiemployer plans, contributions by employer, percent Multiemployer Plans, Contributions By Employer, Percent Multiemployer Plans, Contributions By Employer, Percent Multiemployer plan, contributions by employer Multiemployer Plan, Contributions by Employer Percentage of contributions required for multiple collective-bargaining arrangements Multiemployer Plan, Collective-Bargaining Arrangement, Percentage of Contributions Required for Multiple Collective-Bargaining Arrangements 401 (K) Plan Defined Contribution Plan [Abstract] Employee contribution limit per calendar year Defined Contribution Plan, Maximum Annual Contributions Per Employee, Percent Employer matching contribution, percent of match Defined Contribution Plan, Employer Matching Contribution, Percent of Match Percentage of eligible compensation matched by employer Defined Contribution Plan, Employer Matching Contribution, Percent of Employees' Gross Pay Matching contribution Defined Contribution Plan, Cost Dividend Reinvestment and Stock Purchase Plan (DRIP) Dividend Reinvestment and Stock Purchase Plan [Member] Represents the dividend reinvestment and stock purchase plan of the entity. Issuance of common stock (in shares) Stock Issued During Period, Shares, New Issues Dividend reinvestments/stock purchases under the DRSPP Proceeds from Issuance of Common Stock Interest expense Interest Income (Expense), Net [Abstract] Interest expense before capitalized interest Interest Costs Incurred Interest on financing leases Finance Lease, Interest Expense Interest capitalized Interest Costs Capitalized Adjustment Interest income Interest Income, Operating Interest expense, net Interest Income (Expense), Net Statement of Comprehensive Income [Abstract] Other comprehensive (loss) income: Other Comprehensive Income (Loss), Net of Tax [Abstract] (Decrease) increase in unrealized value of derivative instruments, including SLGOP's share of joint venture derivative instruments Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), after Reclassification and Tax Increase (decrease) in unrealized value of marketable securities Other Comprehensive Income (Loss), Securities, Available-for-sale, Adjustment, after Tax Other comprehensive loss Other Comprehensive Income (Loss), Net of Tax Comprehensive income Comprehensive Income (Loss), Net of Tax, Including Portion Attributable to Noncontrolling Interest Net (income) loss attributable to noncontrolling interests and preferred units distributions Net Income (Loss) Attributable to Noncontrolling Interest Net loss attributable to noncontrolling interests Other comprehensive loss attributable to noncontrolling interests Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Noncontrolling Interest Comprehensive income attributable to SL Green/SLGOP Comprehensive Income (Loss), Net of Tax, Attributable to Parent Real Estate [Table] Real Estate [Table] Property, Plant and Equipment, Type [Axis] Property, Plant and Equipment, Type [Axis] Property, Plant and Equipment, Type [Domain] Property, Plant and Equipment, Type [Domain] Building (fee ownership) Building Fee Ownership [Member] Represents information pertaining to long-lived, depreciable structures held for productive use, including office, production, storage and distribution facilities, on a fee ownership basis. Building (leasehold interest) Building Leasehold Interest [Member] Represents information pertaining to long-lived, depreciable structures held for productive use, including office, production, storage and distribution facilities, in which the entity has a leasehold interest. Right of use assets - financing leases Right Of Use Assets Financing Lease [Member] Right Of Use Assets Financing Lease [Member] Furniture and fixtures Furniture and Fixtures [Member] Building Building [Member] Finite-Lived Intangible Assets by Major Class [Axis] Finite-Lived Intangible Assets by Major Class [Axis] Finite-Lived Intangible Assets, Major Class Name [Domain] Finite-Lived Intangible Assets, Major Class Name [Domain] Above Market Leases Above Market Leases [Member] Below Market Leases Leases, Below Market [Member] Leases, Below Market [Member] In Place Leases In Place Leases [Member] Represents information pertaining to in-place leases. Other Identifiable Assets Other Identifiable Assets [Member] Represents all identifiable assets other than acquired above-market and below-market leases. Acquired above-market leases, net of acquired below-market leases Lease Agreements [Member] Real Estate [Line Items] Real Estate [Line Items] Estimated useful life (in years) Property, Plant and Equipment, Useful Life Estimated useful life of other intangible assets (in years) Finite-Lived Intangible Asset, Useful Life Weighted average amortization period, above-market lease costs Weighted Average Amortization Period, Above-Market Lease Costs Weighted Average Amortization Period, Above-Market Lease Costs Weighted average amortization period, below-market lease costs Weighted Average Amortization Period, Below-Market Lease Costs Weighted Average Amortization Period, Below-Market Lease Costs Weighted average amortization period, in-place lease costs Weighted Average Amortization Period, In-Place Lease Costs Weighted Average Amortization Period, In-Place Lease Costs Period from cessation of major construction activity to completion of construction project, maximum Period from Cessation of Major Construction Activity to Completion of Construction Project, Maximum Represents the maximum period of time from the cessation of major construction activity to the point at which the construction project is considered substantially completed and held available for occupancy and the capitalization of costs is ceased. Depreciation expense (including amortization of the capital lease asset) Commercial Real Estate Properties Depreciation and Amortization The current period expense charged against earnings on long-lived, physical assets not used in production, and which are not intended for resale, to allocate or recognize the cost of such assets over their useful lives; or to record the reduction in book value of an intangible asset over the benefit period of such asset; or to reflect consumption during the period of an asset that is not used in production related to Commercial Real Estate Properties. Amortization of above and below Market Leases Rental Revenue Increase (Decrease) Acquired Lease Amortization Represents the increase (decrease) in rental revenue during the period resulting from the net amortization of acquired above-market and below-market leases. Reduction in interest expense from amortization of above-market rate mortgages Interest Expense Increase (Decrease) Assumed Above Market Rate Mortgage Amortization Represents the increase (decrease) in interest expense during the period resulting from the amortization of assumed above-market rate mortgages. Identified intangible assets (included in other assets): Finitely Lived Intangible Assets [Abstract] -- None. No documentation exists for this element. -- Gross amount Finite-Lived Intangible Assets, Gross Accumulated amortization Finite-Lived Intangible Assets, Accumulated Amortization Net Finite-Lived Intangible Assets, Net Identified intangible liabilities (included in deferred revenue): Finite Lived Intangible Liabilities [Abstract] -- None. No documentation exists for this element. -- Gross amount Below Market Lease, Gross Accumulated amortization Below Market Lease, Accumulated Amortization Net Below Market Lease, Net Net intangible assets reclassed to assets held for sale Net Intangible Assets Transferred to Assets Held for Sale Net Intangible Assets Transferred to Assets Held for Sale Net intangible liabilities reclassed to liabilities relates to assets held for sale Net Intangible Liabilities Transferred to Liabilities Related to Assets Held for Sale Net Intangible Liabilities Transferred to Liabilities Related to Assets Held for Sale Below Market Lease, Net, Amortization Income, Fiscal Year Maturity [Abstract] Below Market Lease, Net, Amortization Income, Fiscal Year Maturity [Abstract] 2020 Below Market Lease, Amortization Income, Next Twelve Months 2021 Below Market Lease, Amortization Income, Year Two 2022 Below Market Lease, Amortization Income, Year Three 2023 Below Market Lease, Amortization Income, Year Four 2024 Below Market Lease, Amortization Income, Year Five Estimated annual amortization Finite-Lived Intangible Assets, Net, Amortization Expense, Fiscal Year Maturity [Abstract] 2020 Finite-Lived Intangible Assets, Amortization Expense, Next Twelve Months 2021 Finite-Lived Intangible Assets, Amortization Expense, Year Two 2022 Finite-Lived Intangible Assets, Amortization Expense, Year Three 2023 Finite-Lived Intangible Assets, Amortization Expense, Year Four 2024 Finite-Lived Intangible Assets, Amortization Expense, Year Five Fair Value Measurements Fair Value Disclosures [Text Block] Changes in real estate SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate [Roll Forward] Balance at beginning of year Property acquisitions SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate, Other Acquisition Improvements SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate, Improvements Retirements/disposals/deconsolidation Real Estate Cost of Real Estate Sold Retired or Deconsolidated The carrying amount of real estate sold, retired or deconsolidated during the period. Balance at end of year Aggregate cost of land, buildings and improvements before depreciation SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate, Federal Income Tax Basis Changes in accumulated depreciation SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate, Accumulated Depreciation [Roll Forward] Balance at beginning of year Depreciation for year SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate, Accumulated Depreciation, Depreciation Expense Retirements/disposals/deconsolidation Real Estate, Accumulated Depreciation Real Estate Sold Retired or Deconsolidated The amount that was removed from accumulated depreciation pertaining to real estate that was sold, retired or deconsolidated in the period. Balance at end of year 717 Fifth Avenue Fifth Avenue, 717 [Member] Represents the information pertaining to 717 Fifth Avenue, a joint venture property of the entity. 650 Fifth Avenue 650 Fifth Avenue [Member] 650 Fifth Avenue [Member] 21 East 66th Street 800 Third Avenue Third Avenue, 800 [Member] Represents the information pertaining to 800 Third Avenue, a joint venture property of the entity. 400 East 57th Street East 400 Street 57 [Member] Represents information pertaining to 400 East 57th Street, a property owned by the entity. Worldwide Plaza Worldwide Plaza [Member] Worldwide Plaza [Member] Stonehenge Portfolio Stonehenge Properties [Member] Represents the information pertaining to Stonehenge Properties. 521 Fifth Avenue 10 East 53rd Street East 53rd Street 10 [Member] Represents information pertaining to 10 East 53rd Street, a joint venture property of the entity. 280 Park Avenue Park Avenue, 280 [Member] Represents the information pertaining to 280 Park Avenue, a joint venture property of the entity. 1552 Broadway Broadway 1552 to 1560 [Member] Represents the information pertaining to 1552-1560 Broadway, a joint venture property of the entity. 121 Greene Street Greene Street 121 [Member] Greene Street 121 [Member] 11 West 34th Street West 11 Street 34 [Member] Represents the information pertaining to 11 West 34th Street, a joint venture property of the entity. 100 Park Avenue Park Avenue, 100 [Member] Represents the information pertaining to 100 Park Avenue, a joint venture property of the entity. One Vanderbilt One Vanderbilt [Member] One Vanderbilt [Member] 55 West 46th Street West 46th Street 55 [Member] West 46th Street 55 [Member] 115 Spring Street 605 West 42nd Street 605 West 42nd Street [Member] 605 West 42nd Street [Member] 131-137 Spring Street 103 East 86th Street 103 East 86th Street [Member] 103 East 86th Street [Member] Initial Maturity August 2019 Secured Debt, Initial Maturity August 2019 [Member] Secured Debt, Initial Maturity August 2019 [Member] Initial Maturity June 2024 Secured Debt, Initial Maturity June 2024 [Member] Secured Debt, Initial Maturity June 2024 [Member] Construction Loans Construction Loans [Member] Mortgage loan Mortgages [Member] Mezzanine loans Mezzanine Loans on Real Estate [Member] Represents the mezzanine loans on real estate properties. Economic Interest, percent Equity Method Investment, Economic Interest Percentage The percentage of economic interest in the investee accounted for under the equity method of accounting. Interest rate, fixed rate debt (as a percent) Long-term Debt, Percentage Bearing Fixed Interest, Percentage Rate Interest rate, floating rate debt (as a percent) Long-term Debt, Percentage Bearing Variable Interest, Percentage Rate Total fixed rate debt Secured Debt, Bearing Fixed Interest, Amount The portion of the carrying amount of long-term secured loans outstanding as of the balance sheet date, including current maturities, which accrues interest at a set, unchanging rate. Total floating rate debt Secured Debt, Bearing Variable Interest, Amount The portion of the carrying amount of long-term secured loans outstanding as of the balance sheet date, including current maturities, which accrues interest at a rate subject to change from time to time. Total fixed rate and floating rate debt Secured Debt and Other Loans, Payable Represents the carrying value as of the balance sheet date, including the current and noncurrent portions, of collateralized debt obligations and other loans payable. Deferred financing costs, net Debt Issuance Costs, Net Total joint venture mortgages and other loans payable, net Secured Debt and Other Loans, Payable, Net Secured Debt and Other Loans, Payable, Net Maximum facility capacity Term (in Years) Debt Instrument, Term Number of extension options Debt Instrument, Number of Extension Options Debt Instrument, Number of Extension Options Period of extension option Debt Instrument, Period of Extension Option Debt Instrument, Period of Extension Option Face amount of loan Debt Instrument, Face Amount Committed amount Debt Instrument, Committed Amount Represents the maximum committed amount of the debt instrument. Debt instrument unfunded amount Debt Instrument Unfunded Amount Debt Instrument Unfunded Amount Derivative Instruments and Hedging Activities Disclosure [Abstract] Financial Instruments: Derivatives and Hedging Derivative Instruments and Hedging Activities Disclosure [Text Block] Schedule of Real Estate Properties [Table] Schedule of Real Estate Properties [Table] Manhattan Manhattan [Member] Represents information pertaining to properties located in Manhattan. Suburban Unconsolidated properties Office Office Building [Member] Retail Retail Site [Member] Development/Redevelopment Fee Interest Fee Interest [Member] Fee Interest [Member] Commercial properties Commercial Real Estate [Member] Residential Apartment Building [Member] Dual property type, retail portion Dual Property Type, Retail Portion [Member] Dual Property Type, Retail Portion [Member] Dual property type, residential portion Dual Property Type, Residential Portion [Member] Dual Property Type, Residential Portion [Member] Managed office properties Managed Office Properties [Member] Represents the office properties managed by the entity. Real estate properties Real Estate Properties [Line Items] Number of Properties Number of Real Estate Properties Approximate Square Feet Weighted Average Occupancy unaudited (as a percent) Real Estate Weighted Average Occupancy Represents the weighted average occupancy of real estate properties owned, which represents the total leased square feet divided by the total available rentable square feet. Debt and preferred equity investments including investments held by unconsolidated joint ventures SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate Debt and preferred equity investments and other financing receivables included in other balance sheet items Debt Investments, Other Debt Investments, Other Number of shares to be received on redemption of one unit of limited partnership interests (shares) Number of Shares on Redemption of One Unit of Partnership Interest Represents the number of shares of the common stock of the entity, which the limited partner of the operating partnership is entitled to receive on redemption of one unit of interest in the partnership. Share-based Compensation Share-based Payment Arrangement [Text Block] Schedule of Deferred Costs Schedule of Deferred Costs [Table Text Block] Tabular disclosure of the components of deferred costs. Summary of properties sold and income from discontinued operations Disposal Groups, Including Discontinued Operations [Table Text Block] Deferred Costs Deferred Financing and Leasing Costs Disclosure [Text Block] Costs incurred in connection with financing activities (fees to banks) and costs incurred in connection with leasing activities (fees to real estate brokers), net of amortization. Schedule of Business Acquisitions, by Acquisition Schedule of Business Acquisitions, by Acquisition [Table Text Block] Quarterly Financial Data of the Company/Operating Partnership (unaudited) Quarterly Financial Information [Text Block] Accumulated Other Comprehensive Income Comprehensive Income (Loss) Note [Text Block] Schedule of senior unsecured notes and other related disclosures by scheduled maturity date Schedule of Long-term Debt Instruments [Table Text Block] Schedule of combined aggregate principal maturities Schedule of Maturities of Long-term Debt [Table Text Block] Schedule of consolidated interest expense, excluding capitalized interest Interest Income and Interest Expense Disclosure [Table Text Block] Cover page. Document Information [Table] Document Information [Table] Document Information [Line Items] Document Information [Line Items] Document Type Document Type Document Annual Report Document Annual Report Document Period End Date Document Period End Date Document Transition Report Document Transition Report Entity File Number Entity File Number Entity Registrant Name Entity Registrant Name Entity Incorporation, State or Country Code Entity Incorporation, State or Country Code Entity Tax Identification Number Entity Tax Identification Number Entity Address, Address Line One Entity Address, Address Line One Entity Address, City or Town Entity Address, City or Town Entity Address, State or Province Entity Address, State or Province Entity Address, Postal Zip Code Entity Address, Postal Zip Code City Area Code City Area Code Local Phone Number Local Phone Number Trading Symbol Trading Symbol Title of 12(b) Security Title of 12(b) Security Security Exchange Name Security Exchange Name Entity Well-known Seasoned Issuer Entity Well-known Seasoned Issuer Entity Voluntary Filers Entity Voluntary Filers Entity Current Reporting Status Entity Current Reporting Status Entity Interactive Data Current Entity Interactive Data Current Entity Filer Category Entity Filer Category Entity Small Business Entity Small Business Entity Emerging Growth Company Entity Emerging Growth Company Entity Shell Company Entity Shell Company Entity Common Stock, Shares Outstanding Entity Common Stock, Shares Outstanding Entity Public Float Entity Public Float Entity Central Index Key Entity Central Index Key Current Fiscal Year End Date Current Fiscal Year End Date Document Fiscal Year Focus Document Fiscal Year Focus Document Fiscal Period Focus Document Fiscal Period Focus Amendment Flag Amendment Flag Documents Incorporated by Reference Documents Incorporated by Reference [Text Block] Partners' Capital of the Operating Partnership Commercial mortgage-backed securities Investment in Marketable Securities Marketable Securities Cost basis of commercial mortgage backed securities Debt Securities, Available-for-sale, Amortized Cost Proceeds from sale of marketable securities Proceeds from Sale of Available-for-sale Securities Realized gain (loss) on investment in marketable securities Marketable Securities, Realized Gain (Loss) Schedule of estimated useful lives Property, Plant and Equipment [Table Text Block] Summary of identified intangible assets (acquired above-market leases and in-place leases) and intangible liabilities (acquired below-market leases) Schedule of Intangible Assets and Liabilities [Table Text Block] Tabular disclosure of the aggregate amount of intangible assets and liabilities. Schedule of estimated annual amortization of acquired above-market leases, net of acquired below-market leases Schedule of Acquired above Market and below Market Leases, Future Amortization Expense [Table Text Block] Tabular disclosure of the amount of amortization expense expected to be recorded in succeeding fiscal years for acquired below-market leases, net of acquired above-market leases. Schedule of estimated annual amortization of all other identifiable assets Schedule of Other Identifiable Assets, Future Amortization Expense [Table Text Block] Tabular disclosure of the amount of amortization expense expected to be recorded in succeeding fiscal years for all identifiable assets other than acquired below-market leases and acquired above-market leases. Schedule of marketable securities Debt Securities, Available-for-sale [Table Text Block] Schedules of concentration of risk, by risk factor Schedules of Concentration of Risk, by Risk Factor [Table Text Block] Mortgages and other loans payable Mortgage and Other Loans Payable [Member] Represents mortgage and other loans payable of the entity. Trust Preferred Securities Junior Subordinated Debt [Member] Revolving Credit Facility Credit Facility 2012 [Member] Represents information pertaining to 2012 Credit Facility. Unsecured Term Loans Senior Unsecured Notes Senior Notes [Member] Debt Instrument [Line Items] Proceeds from issuance of debt Proceeds from Issuance of Debt Scheduled Amortization Future Amortization of Debt [Abstract] -- None. No documentation exists for this element. -- 2020 Future Amortization of Debt, First Full Fiscal Year The amount of amortization of debt expected to be recognized during the first full fiscal year following the date of the most recent balance sheet. 2021 Future Amortization of Debt, Second Full Fiscal Year The amount of amortization of debt expected to be recognized during the second full fiscal year following the date of the most recent balance sheet. 2022 Future Amortization of Debt, Third Full Fiscal Year The amount of amortization of debt expected to be recognized during the third full fiscal year following the date of the most recent balance sheet. 2023 Future Amortization of Debt Fourth Full Fiscal Year The amount of amortization of debt expected to be recognized during the fourth full fiscal year following the date of the most recent balance sheet. 2024 Future Amortization of Debt Fifth Full Fiscal Year The amount of amortization of debt expected to be recognized during the fifth full fiscal year following the date of the most recent balance sheet. Thereafter Future Amortization of Debt after Fifth Full Fiscal Year The amount of amortization of debt expected to be recognized after the fifth full fiscal year following the date of the most recent balance sheet. Principal Repayments and Joint Venture Debt Maturities of Long-term Debt [Abstract] 2020 Long-term Debt, Maturities, Repayments of Principal in Next Twelve Months 2021 Long-term Debt, Maturities, Repayments of Principal in Year Two 2022 Long-term Debt, Maturities, Repayments of Principal in Year Three 2023 Long-term Debt, Maturities, Repayments of Principal in Year Four 2024 Long-term Debt, Maturities, Repayments of Principal in Year Five Thereafter Long-term Debt, Maturities, Repayments of Principal after Year Five Future Amortization of Debt Future Amortization of Debt The total expected future amortization of debt. Repayments of principal Long Term Debt, Maturities, Repayments of Principal Total amount of long-term debt maturing in future periods. Scheduled Amortization and Principal Repayments Amortization and Maturities of Long-term Debt [Abstract] -- None. No documentation exists for this element. -- Total fixed rate and floating rate debt Long-term Debt, Gross Schedule of first mortgages and other loans payable collateralized by the respective properties and assignment of leases Mortgage And Other Loans Payable [Table Text Block] Mortgage And Other Loans Payable [Table Text Block] Third Avenue, 919 11 West 34th Street 280 Park Avenue 333 East 22nd St 333 East 22nd St [Member] 333 East 22nd St [Member] 1552 Broadway Broadway 1552 [Member] Represents information pertaining to 1552 Broadway, a joint venture property of the entity. Real Estate Properties, Type of Property [Axis] Real Estate Properties, Type of Property [Axis] Represents details pertaining to the types of real estate properties. Real Estate Properties, Type of Property [Domain] Real Estate Properties, Type of Property [Domain] Identifies real estate properties by type. Three Retail and Two Residential Units Three Retail and Two Residential Units [Member] Three Retail and Two Residential Units [Member] Three Residential Units Three Residential Units [Member] Three Residential Units [Member] General information on each joint venture Equity method investments with negative book value Equity Method Investments With Negative Book Value Equity Method Investments With Negative Book Value Net equity investment in VIEs Variable Interest Entity, Nonconsolidated, Carrying Amount, Assets and Liabilities, Net Approximate Square Feet Number of Stores Number of Stores Number of residential units Number Of Residential Units Number Of Residential Units Investment [Axis] Investment Type [Axis] Investment [Domain] Investments [Domain] SEC Schedule, 12-29, Real Estate Companies, Investment in Movement in Mortgage Loans on Real Estate [Roll Forward] SEC Schedule, 12-29, Real Estate Companies, Investment in Movement in Mortgage Loans on Real Estate [Roll Forward] Balance at beginning of period Originations/accretion SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate, New Mortgage Loan Redemptions/sales/syndications/amortization SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate, Amortization of Premium Net change in loan loss reserves SEC Schedule, 12-29, Real Estate Companies, Investment In Mortgage Loans On Real Estate, Loan Loss Reserve Period Increase (Decrease) SEC Schedule, 12-29, Real Estate Companies, Investment In Mortgage Loans On Real Estate, Loan Loss Reserve Period Increase (Decrease) Balance at end of period Total revenues Total expenses Equity in net gain on sale of interest in unconsolidated joint venture/real estate (Loss) gain on sale of real estate, net Noncontrolling interests and preferred unit distributions Perpetual preferred stock dividends Preferred Stock Dividends and Other Adjustments Net income attributable to common unitholders per common share—basic (in dollars per share) Net income attributable to common unitholders per common share—diluted (in dollars per share) Accumulated Other Comprehensive Income (Loss) [Table] Accumulated Other Comprehensive Income (Loss) [Table] Net unrealized (loss) gain on derivative instruments Accumulated Gain (Loss), Cash Flow Hedge, Including Noncontrolling Interest [Member] Net unrealized gain on marketable securities AOCI, Accumulated Gain (Loss), Debt Securities, Available-for-sale, Including Noncontrolling Interest [Member] Accumulated Other Comprehensive Income AOCI Including Portion Attributable to Noncontrolling Interest [Member] Accumulated Other Comprehensive Income (Loss) [Line Items] Accumulated Other Comprehensive Income (Loss) [Line Items] Accumulated Other Comprehensive Income (Loss) of the Company [Roll Forward] Accumulated Other Comprehensive Income (Loss) of the Company [Roll Forward] Accumulated Other Comprehensive Income (Loss) of the Company [Roll Forward] Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest Other comprehensive (loss) income before reclassifications Other Comprehensive Income (Loss), before Reclassifications, Net of Tax Amounts reclassified from accumulated other comprehensive loss Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax Deferred net losses from terminated hedges Other Comprehensive Income (Loss), Deferred Net Loss from Terminated Hedges Other Comprehensive Income (Loss), Deferred Net Loss from Terminated Hedges Schedule of Business Acquisitions, by Acquisition [Table] Schedule of Business Acquisitions, by Acquisition [Table] Business Acquisition [Line Items] Business Acquisition [Line Items] Gross Asset Valuation (in millions) Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets Interest acquired Business Acquisition, Percentage of Voting Interests Acquired Fair value adjustment Business Combination Fair Value Adjustment Business Combination Fair Value Adjustment net intangible lease liabilities recognized Business Combination Recognized Identifiable Asset Acquired and Liability Assumed Lease Obligation Business Combination Recognized Identifiable Asset Acquired and Liability Assumed Lease Obligation Series G Preferred Units Series G Preferred Stock [Member] Series F Preferred Units Series F Preferred Stock [Member] Series K Preferred Units Series K Preferred Stock [Member] Series K Preferred Stock [Member] Series L Preferred Units Series L Preferred Stock [Member] Series L Preferred Stock [Member] Series M Preferred Units Series M Preferred Stock [Member] Series M Preferred Stock [Member] Series N Preferred Units Series N Preferred Stock [Member] Series N Preferred Stock [Member] Series O Preferred Units Series O Preferred Stock [Member] Series O Preferred Stock [Member] Series P Preferred Units Series P Preferred Stock [Member] Series P Preferred Stock [Member] Series Q Preferred Units Series Q Preferred Stock [Member] Series Q Preferred Stock [Member] Series R Preferred Units Series R Preferred Stock [Member] Series R Preferred Stock [Member] Series S Preferred Units Series S Preferred Stock [Member] Series S Preferred Stock [Member] Series T Preferred Stock Series T Preferred Stock [Member] [Member] Series T Preferred Stock [Member] [Member] Series U Preferred Stock Series U Preferred Stock [Member] Series U Preferred Stock [Member] Series A Preferred Units Series A Preferred Stock [Member] Series V Preferred Stock Series V Preferred Stock [Member] Series V Preferred Stock [Member] Series B Preferred Stock Series B Preferred Stock [Member] Organization Number of units of operating partnership owned by the noncontrolling interest unit holders (shares) Shares of common stock reserved for issuance upon redemption of units of limited partnership interest in operating partnership (shares) Common Stock Capital Shares Reserved Issuance upon Redemption of Units of Limited Partnership Represents the number of common shares reserved for issuance upon redemption of units of the limited partnership interest. Rollforward analysis of the activity relating to the noncontrolling interests in the operating partnership Stockholders' Equity Attributable to Noncontrolling Interest [Roll Forward] Balance at beginning of period Noncontrolling Interest in Operating Partnerships Distributions Issuance of common units Noncontrolling Interest, Increase from Subsidiary Equity Issuance Net income Accumulated other comprehensive income allocation Fair value adjustment Minority Interest Increase (Decrease) from Fair Value Adjustment Represents the increase (decrease) in noncontrolling interest resulting from fair value adjustments. Balance at end of period Preferred Units, shares authorized (in shares) Preferred Units, Authorized Number of preferred units issued (in shares) Temporary Equity, Shares Issued Annual dividends on preferred units (in dollars per share) Preferred Stock, Dividends, Per Share, Cash Paid Liquidation preference of preferred units (in dollars per share) Conversion Price Per Unit (in dollars per share) Operating Partnership Common Unit Value, Per Share Use for Conversion Represents the common stock value per share which is used for purposes of determining the number of operating partnership common stock issue on conversion of preferred unit. Number of company common stock issue on redemption of operation partnership common units Operating Partnership Common Unit, Convertible Conversion Ratio Operating Partnership Common Unit, Convertible Conversion Ratio Preferred stock, dividend, multiplier per common unit distribution Preferred Stock Dividend Multiplier Ratio Per Common Unit Distribution Preferred Stock Dividend Multiplier Ratio Per Common Unit Distribution Dividend rate preferred units (as a percent) Number of company common stock issued on conversion of Series B preferred units Preferred Stock, Convertible Conversion Ratio Preferred Stock, Convertible Conversion Ratio Preferred units, issued (in shares) Derivative [Table] Derivative [Table] Income Statement Location [Axis] Income Statement Location [Axis] Income Statement Location [Domain] Income Statement Location [Domain] Interest Expense Interest Expense [Member] Income Loss From Equity Method Investments Income Loss From Equity Method Investments [Member] Income Loss From Equity Method Investments [Member] Derivative Instrument [Axis] Derivative Instrument [Axis] Derivative Contract Type [Domain] Derivative Contract [Domain] Interest Rate Swap Expiring May 2020 Interest Rate Swap Expiring May 2020 [Member] Interest Rate Swap Expiring May 2020 [Member] Interest Rate Swap Expiring November 2020 Interest Rate Swap Expiring November 2020 [Member] Interest Rate Swap Expiring November 2020 [Member] Interest Rate Swap Expiring November 2020, 2 Interest Rate Swap Expiring November 2020, 2 [Member] Interest Rate Swap Expiring November 2020, 2 [Member] Interest Rate Swap Expiring December 2020 Interest Rate Swap Expiring December 2020 [Member] Interest Rate Swap Expiring December 2020 [Member] Interest Rate Swap Expiring March 2021 Interest Rate Swap Expiring March 2021 [Member] Interest Rate Swap Expiring March 2021 [Member] Interest Rate Swap Expiring in July 2023 Interest Rate Swap Expiring in July 2023 [Member] Interest Rate Swap Expiring in July 2023 [Member] Interest Rate Swap Expiring in July 2023, 2 Interest Rate Swap Expiring in July 2023, 2 [Member] Interest Rate Swap Expiring in July 2023, 2 [Member] Interest Rate Swap Expiring January 2024 Interest Rate Swap Expiring January 2024 [Member] Interest Rate Swap Expiring January 2024 [Member] Interest Rate Swap Expiring January 2026 Interest Rate Swap Expiring January 2026 [Member] Interest Rate Swap Expiring January 2026 [Member] Interest Rate Swap Expiring January 2026, 2 Interest Rate Swap Expiring January 2026, 2 [Member] Interest Rate Swap Expiring January 2026, 2 [Member] Interest Rate Swap Interest Rate Contract [Member] Financial Instruments: Derivatives and Hedging Derivative [Line Items] Notional Value Derivative Asset, Notional Amount Strike Rate Derivative, Fixed Interest Rate Fair Value Gain (loss) from changes in fair value Accumulated Other Comprehensive Income (Loss), Gain (Loss) from Changes in Fair Value Accumulated Other Comprehensive Income (Loss), Gain (Loss) from Changes in Fair Value Fair value of derivatives in a net liability position Derivative, Net Liability Position, Aggregate Fair Value Aggregate termination value Assets Needed for Immediate Settlement, Aggregate Fair Value Estimated current balance held in accumulated other comprehensive loss to be reclassified into earnings within the next 12 months Cash Flow Hedge Gain (Loss) to be Reclassified within Twelve Months Amount of Loss Recognized in Other Comprehensive Loss Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), before Reclassification and Tax Amount of (Loss) Gain Reclassified from Accumulated Other Comprehensive Loss into Income Interest Rate Cash Flow Hedge Gain (Loss) Reclassified to Earnings, Net Schedule of notional and fair value of derivative financial instruments and foreign currency hedges Schedule of Derivative Instruments [Table Text Block] Schedule of effect of derivative financial instruments on consolidated statements of income Derivative Instruments, Gain (Loss) [Table Text Block] Schedule of Operating Leased Assets [Table] Schedule of Operating Leased Assets [Table] Property Subject to or Available for Operating Lease [Axis] Property Subject to or Available for Operating Lease [Axis] Property Subject to or Available for Operating Lease [Domain] Property Subject to or Available for Operating Lease [Domain] Lessor, Lease, Description [Line Items] Lessor, Lease, Description [Line Items] 2020 Lessor, Operating Lease, Payments to be Received, Next Twelve Months 2021 Lessor, Operating Lease, Payments to be Received, Two Years 2022 Lessor, Operating Lease, Payments to be Received, Three Years 2023 Lessor, Operating Lease, Payments to be Received, Four Years 2024 Lessor, Operating Lease, Payments to be Received, Five Years Thereafter Lessor, Operating Lease, Payments to be Received, Thereafter Total payments to be received Lessor, Operating Lease, Payments to be Received Schedule of commercial office properties Schedule of Real Estate Properties [Table Text Block] Senior Mortgage Debt Senior Mortgage [Member] Senior Mortgage [Member] Junior Mortgage Debt Junior Mortgage [Member] Junior Mortgage [Member] Mezzanine Debt Mezzanine Debt [Member] Mezzanine Debt [Member] Preferred Equity Preferred Equity [Member] Preferred Equity [Member] Face Value Investment Owned, Face Amount Senior Financing Mortgage Loans on Real Estate, Senior Financing Mortgage Loans on Real Estate, Senior Financing Interest rate Investment Interest Rate Basis spread on variable rate Loans Receivable, Basis Spread on Variable Rate Rental Income Lessor, Operating Leases [Text Block] Schedule of general information on joint ventures Equity Method Investments [Table Text Block] Schedule of first mortgage notes payable collateralized by the respective joint venture properties and assignment of leases Schedule of Mortgage and Other Loans Payable on Joint Venture Properties [Table Text Block] Tabular disclosure of the information pertaining to mortgage and other loans payable on joint venture properties of the entity. Schedule of combined balance sheets for the unconsolidated joint ventures Equity Method Investments, Summarized Financial Information Balance Sheet [Table Text Block] Disclosure of summarized balance sheet financial information for investments accounted for using the equity method of accounting. Schedule of combined statements of income for the unconsolidated joint ventures Equity Method Investments, Summarized Financial Information Income Statement [Table Text Block] Disclosure of summarized income statement information for investments accounted for using the equity method of accounting. Schedule of Preferred Units [Table] Schedule of Preferred Units [Table] Preferred Units [Line Items] Preferred Units [Line Items] Rollforward Analysis of Preferred Unit Activity Rollforward Analysis of Preferred Unit Activity [Roll Forward] Rollforward Analysis of Preferred Unit Activity [Roll Forward] Balance at beginning of period Noncontrolling Interest in Preferred Unit Holders Issuance of preferred units Redemption of preferred units Balance at end of period Federal Home Loan Bank, Advances, Option [Axis] Federal Home Loan Bank, Advances, Option [Axis] Federal Home Loan Bank, Advances, Option [Domain] Federal Home Loan Bank, Advances, Option [Domain] LIBOR Plus 25 Basis Points LIBOR Plus 25 Basis Points [Member] LIBOR Plus 25 Basis Points [Member] Ticonderoga Ticonderoga Insurance Company [Member] Ticonderoga Insurance Company [Member] Landmark Square Landmark Square [Member] Represents the information pertaining to Landmark Square, a property owned by the entity. 1080 Amsterdam Avenue Amsterdam Avenue 1080 [Member] Represents information pertaining to property located at 1080 Amsterdam, owned by the entity. A133 Greene Street A133 Greene Street [Member] A133 Greene Street [Member] 460 West 34th Street 460 West 34th Street [Member] 460 West 34th Street [Member] FHLB Facility January 2020 F H L B Facility January 2020 [Member] F H L B Facility January 2020 [Member] FHLB Facility February 2020 F H L B Facility February 2020 [Member] F H L B Facility February 2020 [Member] FHLB Facility June 2020 FHLB Facility June 2020 [Member] FHLB Facility June 2020 [Member] 2017 Master Repurchase Agreement Uncommitted Master Repurchase Agreement 2017 [Member] Uncommitted Master Repurchase Agreement 2017 FHLB Facility, August 2019 FHLB Facility, August 2019 [Member] FHLB Facility, August 2019 [Member] Senior Subordinated Loans Senior Subordinated Loans [Member] FHLB Facility, December 2019 FHLB Facility, December 2019 [Member] FHLB Facility, December 2019 [Member] Weighted Average Weighted Average [Member] Total mortgages and other loans payable Secured Debt, Gross Secured Debt, Gross Deferred financing costs, net of amortization Total mortgages and other loans payable Term (in Years) Interest rate added to base rate (as a percent) Total fixed rate debt Secured Debt, Bearing Fixed Interest, Mortgage Amount Secured Debt, Bearing Fixed Interest, Mortgage Amount Subordinate loan Secured Debt, Bearing Fixed Interest, Subordinate Loan Amount Secured Debt, Bearing Fixed Interest, Subordinate Loan Amount Book value of collateral Collateral Already Posted, Aggregate Fair Value FHLB advances Federal Home Loan Bank, Advances, Branch of FHLB Bank, Amount of Advances Number of extension option Debt Instrument, Extension Option Exercised, Term Debt Instrument, Extension Option Exercised, Term Lessor, Operating Lease, Payments to be Received, Maturity Lessor, Operating Lease, Payments to be Received, Maturity [Table Text Block] Schedule of Future Minimum Rental Payments for Operating Leases Schedule of Future Minimum Rents Receivable for Operating Leases [Table Text Block] Tabular disclosure of future minimum rents receivable in the aggregate and for each of the five succeeding fiscal years for operating leases having initial or remaining noncancelable lease terms in excess of one year. Operating Lease, Lease Income Operating Lease, Lease Income [Table Text Block] Schedule of Segment Reporting Information, by Segment [Table] Schedule of Segment Reporting Information, by Segment [Table] Consolidation Items [Axis] Consolidation Items [Axis] Consolidation Items [Domain] Consolidation Items [Domain] Operating Segments Operating Segments [Member] Business Segments [Axis] Segments [Axis] Segment [Domain] Segments [Domain] Real Estate Segment Real Estate Segment [Member] Represents the information pertaining to the reportable segment of the entity, Real Estate Segment. Debt and Preferred Equity Segment Structured Finance Segment [Member] Represents the information pertaining to the reportable segment of the entity, Structured Finance Segment. Segment information Segment Reporting Information [Line Items] Number of reportable segments (segment) Number of Reportable Segments Total revenues Net Income Income (Loss) from Continuing Operations before Realized Gain (Loss) on Disposal of Equity Method Investment This element represents the income or loss from continuing operations attributable to the economic entity which may also be defined as revenue less expenses and taxes from ongoing operations before realized gains (losses) on disposals of equity method investments, extraordinary items, and noncontrolling interest. Total assets Assets New Accounting Pronouncements or Change in Accounting Principle [Table] New Accounting Pronouncements or Change in Accounting Principle [Table] Accounting Standards Update 2016-02 Accounting Standards Update 2016-02 [Member] New Accounting Pronouncements or Change in Accounting Principle [Line Items] New Accounting Pronouncements or Change in Accounting Principle [Line Items] Lease, cost Lease, Cost Segment Information Segment Reporting Disclosure [Text Block] Schedule of Contributions made to Multi-Employer Plans Schedule of Multiemployer Plans [Table Text Block] Schedule of weighted average assumptions used to estimate the grant date fair value of options granted Schedule of Share-based Payment Award, Stock Options, Valuation Assumptions [Table Text Block] Summary of the status of stock options and changes during the period Share-based Payment Arrangement, Option, Activity [Table Text Block] Schedule of other share-based compensation, activity Share-based Payment Arrangement, Outstanding Award, Activity, Excluding Option [Table Text Block] Equity in net income from unconsolidated joint ventures Net (loss) income attributable to common stockholders per common share - basic (usd per share) Net (loss) income attributable to common stockholders per common share - diluted (usd per share) Schedule of Related Party Transactions, by Related Party [Table] Schedule of Related Party Transactions, by Related Party [Table] Alliance Building Services Alliance Building Services [Member] Represents the information pertaining to Alliance Building Services, which is a related party of the entity. Stephen L Green Interest Related Party Stephen L Green Interest [Member] Represents the information pertaining to an entity in which Stephen L. Green owns an interest, which is a related party of the entity. Marc Holliday, Chairman And CEO Marc Holliday, Chairman And CEO [Member] Marc Holliday, Chairman And CEO [Member] Andrew Mathias, President Andrew Mathias, President [Member] Andrew Mathias, President [Member] Holiday And Mathias Holiday And Mathias [Member] Holiday And Mathias [Member] Affiliated Entity Affiliated Entity [Member] Related Party Transaction [Line Items] Related Party Transaction [Line Items] Profit participation from related party Related Party Transaction, Other Revenues from Transactions with Related Party Property management fees from related party Revenue from Related Parties Percentage of profits due to investors Related Party Transaction, Percentage of Profits Due to Investors Related Party Transaction, Percentage of Profits Due to Investors Due from (to) related party Related Party Transaction, Due from (to) Related Party Stabilization of property, within three years after stabilization, percent Related Party Transaction, Stabilization Of Property, Within Three Years After Stabilization, Percent Related Party Transaction, Stabilization Of Property, Within Three Years After Stabilization, Percent Stabilization of property, three years or more after stabilization, percent Related Party Transaction, Stabilization Of Property, Three Years Or More After Stabilization, Percent Related Party Transaction, Stabilization Of Property, Three Years Or More After Stabilization, Percent Stabilization of property, anniversary period Related Party Transaction, Stabilization Of Property, Anniversary Period Related Party Transaction, Stabilization Of Property, Anniversary Period Amounts due from/to related parties Due from Related Parties, Unclassified [Abstract] Due from joint ventures Due from Joint Ventures Other Due from Other Related Parties Related party receivables Due from Related Parties Schedule of amounts due from/to related parties Schedule of Related Party Transactions [Table Text Block] Senior Unsecured Bonds Senior Unsecured Bonds [Member] Senior Unsecured Bonds Senior unsecured notes Senior Unsecured Notes [Member] Senior Unsecured Notes [Member] 7.75% Senior unsecured notes maturing on March 15, 2020 Senior Unsecured Notes Due March 15, 2020 [Member] Represents senior unsecured notes maturing on March 15, 2020. 0.98% Senior unsecured notes maturing August 2021 Senior Unsecured Notes Due August 2021 [Member] Senior Unsecured Notes Due August 2021 [Member] 3.25% Senior unsecured notes maturing October 2022 3.25 Percent Senior Unsecured Notes Due October 2022 [Member] 3.25 Percent Senior Unsecured Notes Due October 2022 4.50% Senior unsecured notes maturing on December 1, 2022 Senior Unsecured Notes Due December 1, 2022 [Member] Represents senior unsecured notes bearing an interest rate of 4.50 percent maturing on December 1, 2022. 4.27% Senior unsecured notes maturing on December 17, 2025 Senior Unsecured Note Due December 17, 2025 [Member] Senior Unsecured Note Due December 17, 2025 [Member] 4.50% Senior Unsecured Bonds Due December 2022 4.50 Percent Senior Unsecured Bonds Due December 2022 [Member] 4.50 Percent Senior Unsecured Bonds Due December 2022 [Member] Debt disclosures by scheduled maturity date Debt Instrument, Information by Scheduled Maturity Date of Debt [Abstract] -- None. No documentation exists for this element. -- Unpaid Principal Balance Accreted Balance Senior Notes Senior notes, net of deferred finance costs Senior Notes, Net of Deferred Finance Costs Senior Notes, Net of Deferred Finance Costs Coupon Rate (as a percent) Debt Instrument, Interest Rate, Stated Percentage Initial Term (in Years) Redemption price, percentage Debt Instrument, Redemption Price, Percentage Other Commitments [Table] Other Commitments [Table] Belmont Belmont Insurance Company [Member] Represents the information pertaining to Belmont Insurance Company, a wholly-owned taxable REIT subsidiary. Name of Property [Axis] Name of Property [Axis] Name of Property [Domain] Name of Property [Domain] Other Commitments [Line Items] Other Commitments [Line Items] Employment agreements with certain executives, minimum cash-based compensation for next fiscal year Employment Agreements, Minimum Cash Based Compensation in Next Fiscal Year Represents the minimum cash-based compensation, including base salary and guaranteed bonus payments, required in the next fiscal year under employment agreements. Loss reserves Liability for Claims and Claims Adjustment Expense Ownership percentage Initial term of non cancellable operating leases, minimum (in years) Operating Leases, Minimum Lease Term Represents the initial minimum term of the non cancellable operating leases. Financing leases Capital Leases, Future Minimum Payments, Present Value of Net Minimum Payments, Fiscal Year Maturity [Abstract] 2020 Finance Lease, Liability, Payments, Due Next Twelve Months 2021 Finance Lease, Liability, Payments, Due Year Two 2022 Finance Lease, Liability, Payments, Due Year Three 2023 Finance Lease, Liability, Payments, Due Year Four 2024 Finance Lease, Liability, Payments, Due Year Five Thereafter Finance Lease, Liability, Payments, Due after Year Five Total minimum lease payments Finance Lease, Liability, Payment, Due Amount representing interest Finance Lease, Liability, Undiscounted Excess Amount Lease liability - financing leases Non-cancellable operating leases Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract] 2020 Lessee, Operating Lease, Liability, Payments, Due Next Twelve Months 2021 Lessee, Operating Lease, Liability, Payments, Due Year Two 2022 Lessee, Operating Lease, Liability, Payments, Due Year Three 2023 Lessee, Operating Lease, Liability, Payments, Due Year Four 2024 Lessee, Operating Lease, Liability, Payments, Due Year Five Thereafter Lessee, Operating Lease, Liability, Payments, Due after Year Five Total minimum lease payments Lessee, Operating Lease, Liability, Payments, Due Amount discounted using incremental borrowing rate Lessee, Operating Lease, Liability, Undiscounted Excess Amount Lease liability - operating leases Minimum sublease rentals to be received in the future Finance lease cost Finance Lease Cost Finance Lease Cost Finance lease, interest expense Finance lease, right-of-use asset, amortization Finance Lease, Right-of-Use Asset, Amortization Operating lease, cost Operating lease, weighted average discount rate, percent Operating Lease, Weighted Average Discount Rate, Percent Operating lease, weighted average remaining lease term Operating Lease, Weighted Average Remaining Lease Term 1231 Third Avenue 1231 Third Avenue [Member] 1231 Third Avenue [Member] Upper East Side Residential Upper East Side Residential [Member] Upper East Side Residential [Member] Acquisition Price (in millions) Business Combination, Consideration Transferred Number of land parcels, fee interest acquired Fee Interest Acquired, Number Of Land Parcels Fee Interest Acquired, Number Of Land Parcels Assets Assets [Abstract] Commercial real estate properties, at cost: Real Estate Investment Property, at Cost [Abstract] Land and land interests Right of use asset - operating leases Operating Lease, Right-of-Use Asset Total commercial real estate properties, at cost Real Estate Investment Property, at Cost Less: accumulated depreciation Total commercial real estate properties, net Assets held for sale Disposal Group, Including Discontinued Operation, Assets Investments in marketable securities Marketable Securities Tenant and other receivables, net of allowance of $12,369 and $15,702 in 2019 and 2018, respectively Related party receivables Deferred rents receivable, net of allowance of $12,477 and $15,457 in 2019 and 2018, respectively Deferred Rent Receivables, Net Debt and preferred equity investments, net of discounts and deferred origination fees of $14,562 and $22,379 and allowances of $1,750 and $5,750 in 2019 and 2018, respectively Deferred costs, net Total assets Liabilities Liabilities [Abstract] Revolving credit facility, net Unsecured term loans, net Unsecured Debt Unsecured notes, net Accounts payable and accrued expenses Accounts Payable and Accrued Liabilities Deferred revenue Contract with Customer, Liability Dividend and distributions payable Dividends Payable Security deposits Security Deposit Liability Liabilities related to assets held for sale Disposal Group, Including Discontinued Operation, Liabilities Junior subordinated deferrable interest debentures held by trusts that issued trust preferred securities Junior Subordinated Debenture Owed to Unconsolidated Subsidiary Trust Total liabilities Liabilities Commitments and contingencies Commitments and Contingencies Noncontrolling interests in Operating Partnership Preferred units Temporary Equity, Carrying Amount, Attributable to Parent Limited partner interests in SLGOP (4,196 and 4,131 limited partner common units outstanding at December 31, 2019 and 2018, respectively) Limited Partners' Capital Account Preferred units Redeemable Noncontrolling Interest, Equity, Preferred, Carrying Amount Equity Stockholders Equity Including Portion Attributable to Noncontrolling Interest in Other Partnership [Abstract] -- None. No documentation exists for this element. -- Series I Preferred Stock, $0.01 par value, $25.00 liquidation preference, 9,200 issued and outstanding at both December 31, 2019 and 2018 Preferred Stock, Value, Issued Common stock, $0.01 par value, 160,000 shares authorized and 80,257 and 84,739 issued and outstanding at December 31, 2019 and 2018, respectively (including 1,055 and 1,055 shares held in treasury at December 31, 2019 and 2018, respectively) Common Stock, Value, Issued Additional paid-in-capital Additional Paid in Capital Treasury stock at cost Treasury Stock, Value Accumulated other comprehensive (loss) income Accumulated Other Comprehensive Income (Loss), Net of Tax Retained earnings Retained Earnings (Accumulated Deficit) Total SL Green stockholders' equity Stockholders' Equity Attributable to Parent Noncontrolling interests in other partnerships Nonredeemable Noncontrolling Interest Total equity SL Green stockholders equity: Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest [Abstract] Series I Preferred Units, $25.00 liquidation preference, 9,200 issued and outstanding at both December 31, 2019 and 2018 Preferred Units, Preferred Partners' Capital Accounts SL Green partners' capital (834 and 878 general partner common units, and 78,368 and 82,806 limited partner common units outstanding at December 31, 2019 and 2018, respectively) General And Limited Partners Capital Account General And Limited Partners Capital Account Total SLGOP partners' capital Partners' Capital Noncontrolling interests in other partnerships Partners' Capital Attributable to Noncontrolling Interest Total capital Total liabilities and equity/capital Liabilities and Equity Debt and Preferred Equity Investments Loans, Notes, Trade and Other Receivables Disclosure [Text Block] Schedule of debt and preferred equity book balance roll forward Schedule Of Debt And Preferred Equity Book Balance Roll Forward [Table Text Block] Schedule Of Debt And Preferred Equity Book Balance Roll Forward Schedule of Debt Schedule of Debt [Table Text Block] Allowance for credit losses on financing receivables Financing Receivable, Allowance for Credit Loss [Table Text Block] Summary of debt investments Schedule of Accounts, Notes, Loans and Financing Receivable [Table Text Block] Summary of preferred equity investments Investments Classified by Contractual Maturity Date [Table Text Block]