Consolidated Statements of Cash Flows - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Operating Activities
Net income		$ 291,487	$ 270,856	$ 101,069
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization		284,011	289,899	418,798
Equity in net loss (income) from unconsolidated joint ventures		34,518	(7,311)	(21,892)
Distributions of cumulative earnings from unconsolidated joint ventures		864	10,277	20,309
Equity in net gain on sale of interest in unconsolidated joint venture interest/real estate		(76,181)	(303,967)	(16,166)
Purchase price and other fair value adjustments		(69,389)	(57,385)	0
Depreciable real estate reserves and impairment		7,047	227,543	178,520
Loss (gain) on sale of real estate, net		16,749	30,757	(73,241)
Loan loss reserves and other investment reserves, net of recoveries		0	6,839	0
Gain on sale of investments in marketable securities		0	0	(3,262)
Loss on early extinguishment of debt		0	17,083	0
Deferred rents receivable		(13,941)	(18,216)	(38,009)
Non-cash lease expense		13,744	2,016	1,020
Other non-cash adjustments		271	2,932	19,621
Changes in operating assets and liabilities:
Tenant and other receivables		(4,968)	6,968	(5,717)
Related party receivables		7,802	(1,044)	(7,209)
Deferred lease costs		(70,938)	(44,158)	(41,939)
Other assets		(18,630)	(8,310)	(23,068)
Accounts payable, accrued expenses, other liabilities and security deposits		(25,597)	4,410	(12,440)
Deferred revenue		10,824	12,348	46,607
Change in lease liability - operating leases		(11,200)	0	0
Net cash provided by operating activities		376,473	441,537	543,001
Investing Activities
Acquisitions of real estate property		(262,591)	(60,486)	(28,680)
Additions to land, buildings and improvements		(252,986)	(254,460)	(336,001)
Acquisition deposits and deferred purchase price		(5,239)	0	0
Investments in unconsolidated joint ventures		(128,682)	(400,429)	(389,249)
Distributions in excess of cumulative earnings from unconsolidated joint ventures		79,020	233,118	319,745
Net proceeds from disposition of real estate/joint venture interest		208,302	1,231,004	692,796
Proceeds from sale of marketable securities		0	0	55,129
Other investments		(7,869)	(38,912)	25,330
Origination of debt and preferred equity investments		(607,844)	(731,216)	(1,129,970)
Repayments or redemption of debt and preferred equity investments		1,092,383	703,043	812,914
Net cash provided by investing activities		114,494	681,662	22,014
Financing Activities
Proceeds from mortgages and other loans payable		752,984	564,391	870,459
Repayments of mortgages and other loans payable		(230,076)	(868,842)	(902,460)
Proceeds from revolving credit facility and senior unsecured notes		1,310,000	3,120,000	2,784,599
Repayments of revolving credit facility and senior unsecured notes		(1,570,000)	(2,560,000)	(2,276,782)
Payment of debt extinguishment costs		0	(13,918)	0
Proceeds from stock options exercised and DRSPP issuance		334	29,048	23,537
Repurchase of common stock		(384,399)	(979,541)	(806,302)
Redemption of preferred stock		(18,142)	(1,208)	(275)
Redemption of OP units		(27,495)	(33,972)	0
Distributions to noncontrolling interests in other partnerships		(478)	(8,364)	(52,446)
Contributions from noncontrolling interests in other partnerships		10,239	5,459	36,275
Acquisition of subsidiary interest from noncontrolling interest		(25,845)	0	0
Distributions to noncontrolling interests in the Operating Partnership		(14,729)	(15,000)	(14,266)
Dividends/Distributions paid on common and preferred stock/units		(306,386)	(313,230)	(333,543)
Other obligations related to loan participations		0	16	17,227
Tax withholdings related to restricted share awards		(3,495)	(3,842)	(3,879)
Deferred loan costs		(21,162)	(15,109)	(27,100)
Net cash used in financing activities		(528,650)	(1,094,112)	(684,956)
Net (decrease) increase in cash, cash equivalents, and restricted cash		(37,683)	29,087	(119,941)
Cash, cash equivalents, and restricted cash at beginning of year		279,113	250,026	369,967
Cash, cash equivalents, and restricted cash at end of period		241,430	279,113	250,026
Supplemental Cash Flow Information [Abstract]
Interest paid		248,684	259,776	273,819
Income taxes paid		1,489	1,418	2,448
Supplemental Disclosure of Non-Cash Investing and Financing Activities:
Issuance of units in the operating partnership		0	0	25,723
Conversion of units in the operating partnership		471	16,303	21,574
Redemption of units in the operating partnership for a joint venture sale		0	10,445	0
Exchange of debt investment for real estate or equity in joint venture		34,498	298,956	0
Tenant improvements and capital expenditures payable		6,056	0	6,667
Fair value adjustment to noncontrolling interest in the operating partnership		34,320	34,236	5,712
Deconsolidation of a subsidiary	[1]	395	298,404	695,204
Transfer of assets to assets held for sale		391,664	0	611,809
Transfer of liabilities related to assets held for sale		0	0	5,364
Removal of fully depreciated commercial real estate properties		19,577	124,249	15,488
Contribution to consolidated joint venture by noncontrolling interest		48,223	0	0
Share repurchase payable		0	0	41,746
Recognition of right of use assets and related lease liabilities		389,120	0	0
Total cash, cash equivalents, and restricted cash		279,113	279,113	369,967
SL Green Operating Partnership
Operating Activities
Net income		291,487	270,856	101,069
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization		284,011	289,899	418,798
Equity in net loss (income) from unconsolidated joint ventures		34,518	(7,311)	(21,892)
Distributions of cumulative earnings from unconsolidated joint ventures		864	10,277	20,309
Equity in net gain on sale of interest in unconsolidated joint venture interest/real estate		(76,181)	(303,967)	(16,166)
Purchase price and other fair value adjustments		(69,389)	(57,385)	0
Depreciable real estate reserves and impairment		7,047	227,543	178,520
Loss (gain) on sale of real estate, net		16,749	30,757	(73,241)
Loan loss reserves and other investment reserves, net of recoveries		0	6,839	0
Gain on sale of investments in marketable securities		0	0	(3,262)
Loss on early extinguishment of debt		0	17,083	0
Deferred rents receivable		(13,941)	(18,216)	(38,009)
Non-cash lease expense		13,744	2,016	1,020
Other non-cash adjustments		271	2,932	19,621
Changes in operating assets and liabilities:
Tenant and other receivables		(4,968)	6,968	(5,717)
Related party receivables		7,802	(1,044)	(7,209)
Deferred lease costs		(70,938)	(44,158)	(41,939)
Other assets		(18,630)	(8,310)	(23,068)
Accounts payable, accrued expenses, other liabilities and security deposits		(25,597)	4,410	(12,440)
Deferred revenue		10,824	12,348	46,607
Change in lease liability - operating leases		(11,200)	0	0
Net cash provided by operating activities		376,473	441,537	543,001
Investing Activities
Acquisitions of real estate property		(262,591)	(60,486)	(28,680)
Additions to land, buildings and improvements		(252,986)	(254,460)	(336,001)
Acquisition deposits and deferred purchase price		(5,239)	0	0
Investments in unconsolidated joint ventures		(128,682)	(400,429)	(389,249)
Distributions in excess of cumulative earnings from unconsolidated joint ventures		79,020	233,118	319,745
Net proceeds from disposition of real estate/joint venture interest		208,302	1,231,004	692,796
Proceeds from sale of marketable securities		0	0	55,129
Other investments		(7,869)	(38,912)	25,330
Origination of debt and preferred equity investments		(607,844)	(731,216)	(1,129,970)
Repayments or redemption of debt and preferred equity investments		1,092,383	703,043	812,914
Net cash provided by investing activities		114,494	681,662	22,014
Financing Activities
Proceeds from mortgages and other loans payable		752,984	564,391	870,459
Repayments of mortgages and other loans payable		(230,076)	(868,842)	(902,460)
Proceeds from revolving credit facility and senior unsecured notes		1,310,000	3,120,000	2,784,599
Repayments of revolving credit facility and senior unsecured notes		(1,570,000)	(2,560,000)	(2,276,782)
Payment of debt extinguishment costs		0	(13,918)	0
Proceeds from stock options exercised and DRSPP issuance		334	29,048	23,537
Repurchase of common stock		(384,399)	(979,541)	(806,302)
Redemption of preferred stock		(18,142)	(1,208)	(275)
Redemption of OP units		(27,495)	(33,972)	0
Distributions to noncontrolling interests in other partnerships		(478)	(8,364)	(52,446)
Contributions from noncontrolling interests in other partnerships		10,239	5,459	36,275
Acquisition of subsidiary interest from noncontrolling interest		(25,845)	0	0
Dividends/Distributions paid on common and preferred stock/units		(321,115)	(328,230)	(347,809)
Other obligations related to loan participations		0	16	17,227
Tax withholdings related to restricted share awards		(3,495)	(3,842)	(3,879)
Deferred loan costs		(21,162)	(15,109)	(27,100)
Net cash used in financing activities		(528,650)	(1,094,112)	(684,956)
Net (decrease) increase in cash, cash equivalents, and restricted cash		(37,683)	29,087	(119,941)
Cash, cash equivalents, and restricted cash at beginning of year		279,113	250,026	369,967
Cash, cash equivalents, and restricted cash at end of period		241,430	279,113	250,026
Supplemental Cash Flow Information [Abstract]
Interest paid		248,684	259,776	273,819
Income taxes paid		1,489	1,418	2,448
Supplemental Disclosure of Non-Cash Investing and Financing Activities:
Issuance of units in the operating partnership		0	0	25,723
Conversion of units in the operating partnership		471	16,303	21,574
Redemption of units in the operating partnership for a joint venture sale		0	10,445	0
Exchange of debt investment for real estate or equity in joint venture		34,498	298,956	0
Tenant improvements and capital expenditures payable		6,056	0	6,667
Fair value adjustment to noncontrolling interest in the operating partnership		34,320	34,236	5,712
Deconsolidation of a subsidiary	[2]	395	298,404	695,204
Transfer of assets to assets held for sale		391,664	0	611,809
Transfer of liabilities related to assets held for sale		0	0	5,364
Removal of fully depreciated commercial real estate properties		19,577	124,249	15,488
Contribution to consolidated joint venture by noncontrolling interest		48,223	0	0
Share repurchase payable		0	0	41,746
Recognition of right of use assets and related lease liabilities		389,120	0	0
Total cash, cash equivalents, and restricted cash		241,430	250,026	250,026
Preferred Units
Supplemental Disclosure of Non-Cash Investing and Financing Activities:
Issuance of preferred units relating to the real estate acquisition		1,000	0	0
Preferred Units | SL Green Operating Partnership
Supplemental Disclosure of Non-Cash Investing and Financing Activities:
Issuance of preferred units relating to the real estate acquisition		$ 1,000	$ 0	$ 0

[1] $366.6 million of the 2017 amount relates to 1515 Broadway. In November 2017, the Company sold a 30.13% interest in 1515 Broadway to affiliates of Allianz Real Estate. The sale did not meet the criteria for sale accounting and as a result the property was accounted for under the profit sharing method. The Company achieved sale accounting upon adoption of ASC 610-20 in January 2018 and closed on the sale of an additional 12.87% interest in the property to Allianz in February 2018. See Note 6, "Investments in Unconsolidated Joint Ventures.".

[2] $366.6 million of the 2017 amount relates to 1515 Broadway. In November 2017, the Company sold a 30.13% interest in 1515 Broadway to affiliates of Allianz Real Estate. The sale did not meet the criteria for sale accounting and as a result the property was accounted for under the profit sharing method. The Company achieved sale accounting upon adoption of ASC 610-20 in January 2018 and closed on the sale of an additional 12.87% interest in the property to Allianz in February 2018.