Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018	Sep. 30, 2019	Dec. 31, 2018	Sep. 30, 2018
Operating Activities
Net income	$ 40,722	$ 99,454	$ 267,232	$ 329,176
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization			216,834	217,762
Equity in net loss (income) from unconsolidated joint ventures	9,864	(971)	22,644	(9,709)
Distributions of cumulative earnings from unconsolidated joint ventures			648	10,174
Equity in net gain on sale of interest in unconsolidated joint venture interest/real estate	0	(70,937)	(76,181)	(136,522)
Purchase price and other fair value adjustments			(69,389)	(57,385)
Depreciable real estate reserves and impairment			7,047	6,691
Gain on sale of real estate, net			(2,492)	(6,227)
Loan loss reserves and other investment reserves, net of recoveries			0	1,087	$ 6,839
Loss on early extinguishment of debt			0	2,194
Deferred rents receivable			(10,005)	(11,328)
Other non-cash adjustments			(9,573)	8,907
Changes in operating assets and liabilities:
Tenant and other receivables			(7,564)	3,186
Related party receivables			8,921	5,186
Deferred lease costs			(38,542)	(31,828)
Other assets			(47,139)	(52,172)
Accounts payable, accrued expenses, other liabilities and security deposits			18,737	20,715
Deferred revenue and land leases payable			18,255	17,638
Net cash provided by operating activities			299,433	317,545
Investing Activities
Acquisitions of real estate property			(261,596)	(40,279)
Additions to land, buildings and improvements			(150,742)	(163,750)
Acquisition deposits and deferred purchase price			(228)	0
Investments in unconsolidated joint ventures			(114,050)	(298,836)
Distributions in excess of cumulative earnings from unconsolidated joint ventures			62,576	208,724
Net proceeds from disposition of real estate/joint venture interest			138,948	892,350
Other investments			(2,491)	991
Origination of debt and preferred equity investments			(555,951)	(652,309)
Repayments or redemption of debt and preferred equity investments			642,634	553,341
Net cash (used in) provided by investing activities			(240,900)	500,232
Financing Activities
Proceeds from mortgages and other loans payable			699,072	339,125
Repayments of mortgages and other loans payable			(162,024)	(351,080)
Proceeds from revolving credit facility and senior unsecured notes			1,090,000	2,205,000
Repayments of revolving credit facility and senior unsecured notes			(1,255,000)	(2,000,000)
Proceeds from stock options exercised and DRSPP issuance			302	15,689
Repurchase of common stock			(175,005)	(760,073)
Redemption of preferred units			(15,142)	(350)
Redemption of OP units			(15,918)	0
Distributions to noncontrolling interests in other partnerships			(478)	(1,863)
Contributions from noncontrolling interests in other partnerships			4,873	4,900
Acquisition of subsidiary interest from noncontrolling interest			(25,791)	0
Distributions to noncontrolling interests in the Operating Partnership			(10,993)	(11,469)
Dividends / Distributions paid on common and preferred stock / units			(230,804)	(237,007)
Other obligations related to loan participations			0	16
Tax withholdings related to restricted share awards			(3,126)	(3,842)
Deferred loan costs and capitalized lease obligation			(21,068)	(8,257)
Net cash used in financing activities			(121,102)	(809,211)
Net (decrease) increase in cash, cash equivalents, and restricted cash			(62,569)	8,566
Cash, cash equivalents, and restricted cash at end of period	216,544	258,592	216,544	258,592
Supplemental Disclosure of Non-Cash Investing and Financing Activities:
Conversion of units in the Operating Partnership for common stock			471	1,560
Issuance of preferred units relating to a real estate acquisition			1,000	0
Tenant improvements and capital expenditures payable			10,040	16,246
Fair value adjustment to noncontrolling interest in Operating Partnership			15,509	7,448
Deconsolidation of subsidiaries			395	298,391
Transfer of assets related to assets held for sale			403,488	734,008
Transfer of liabilities related to assets held for sale			0	310,496
Removal of fully depreciated commercial real estate properties			6,602	115,320
Contribution to consolidated joint venture by noncontrolling interest			48,223	0
Share repurchase payable			8,839	41,746
Recognition of right of use assets and related lease liabilities			389,120	0
Cash and cash equivalents						$ 121,751	$ 129,475	$ 160,248
Restricted cash						94,793	149,638	98,344
Total cash, cash equivalents, and restricted cash	216,544	258,592	216,544	258,592		216,544		258,592
SL Green Operating Partnership
Operating Activities
Net income	40,722	99,454	267,232	329,176
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization			216,834	217,762
Equity in net loss (income) from unconsolidated joint ventures	9,864	(971)	22,644	(9,709)
Distributions of cumulative earnings from unconsolidated joint ventures			648	10,174
Equity in net gain on sale of interest in unconsolidated joint venture interest/real estate	0	(70,937)	(76,181)	(136,522)
Purchase price and other fair value adjustments			(69,389)	(57,385)
Depreciable real estate reserves and impairment			7,047	6,691
Gain on sale of real estate, net			(2,492)	(6,227)
Loan loss reserves and other investment reserves, net of recoveries			0	1,087
Loss on early extinguishment of debt			0	2,194
Deferred rents receivable			(10,005)	(11,328)
Other non-cash adjustments			(9,573)	8,907
Changes in operating assets and liabilities:
Tenant and other receivables			(7,564)	3,186
Related party receivables			8,921	5,186
Deferred lease costs			(38,542)	(31,828)
Other assets			(47,139)	(52,172)
Accounts payable, accrued expenses, other liabilities and security deposits			18,737	20,715
Deferred revenue and land leases payable			18,255	17,638
Net cash provided by operating activities			299,433	317,545
Investing Activities
Acquisitions of real estate property			(261,596)	(40,279)
Additions to land, buildings and improvements			(150,742)	(163,750)
Acquisition deposits and deferred purchase price			(228)	0
Investments in unconsolidated joint ventures			(114,050)	(298,836)
Distributions in excess of cumulative earnings from unconsolidated joint ventures			62,576	208,724
Net proceeds from disposition of real estate/joint venture interest			138,948	892,350
Other investments			(2,491)	991
Origination of debt and preferred equity investments			(555,951)	(652,309)
Repayments or redemption of debt and preferred equity investments			642,634	553,341
Net cash (used in) provided by investing activities			(240,900)	500,232
Financing Activities
Proceeds from mortgages and other loans payable			699,072	339,125
Repayments of mortgages and other loans payable			(162,024)	(351,080)
Proceeds from revolving credit facility and senior unsecured notes			1,090,000	2,205,000
Repayments of revolving credit facility and senior unsecured notes			(1,255,000)	(2,000,000)
Proceeds from stock options exercised and DRSPP issuance			302	15,689
Repurchase of common stock			(175,005)	(760,073)
Redemption of preferred units			(15,142)	(350)
Redemption of OP units			(15,918)	0
Distributions to noncontrolling interests in other partnerships			(478)	(1,863)
Contributions from noncontrolling interests in other partnerships			4,873	4,900
Acquisition of subsidiary interest from noncontrolling interest			(25,791)	0
Dividends / Distributions paid on common and preferred stock / units			(241,797)	(248,476)
Other obligations related to loan participations			0	16
Tax withholdings related to restricted share awards			(3,126)	(3,842)
Deferred loan costs and capitalized lease obligation			(21,068)	(8,257)
Net cash used in financing activities			(121,102)	(809,211)
Net (decrease) increase in cash, cash equivalents, and restricted cash			(62,569)	8,566
Cash, cash equivalents, and restricted cash at beginning of year			279,113	250,026	250,026
Cash, cash equivalents, and restricted cash at end of period	216,544	258,592	216,544	258,592	279,113
Supplemental Disclosure of Non-Cash Investing and Financing Activities:
Conversion of units in the Operating Partnership for common stock			471	1,560
Redemption of units in the Operating Partnership for the sale of a joint venture			0	10,445
Issuance of preferred units relating to a real estate acquisition			1,000	0
Tenant improvements and capital expenditures payable			10,040	16,246
Fair value adjustment to noncontrolling interest in Operating Partnership			15,509	7,448
Deconsolidation of subsidiaries			395	298,391
Transfer of assets related to assets held for sale			403,488	734,008
Transfer of liabilities related to assets held for sale			0	310,496
Removal of fully depreciated commercial real estate properties			6,602	115,320
Contribution to consolidated joint venture by noncontrolling interest			48,223	0
Share repurchase payable			8,839	41,746
Recognition of right of use assets and related lease liabilities			389,120	0
Cash and cash equivalents						121,751	129,475	160,248
Restricted cash						94,793	149,638	98,344
Total cash, cash equivalents, and restricted cash	$ 216,544	$ 258,592	$ 279,113	$ 250,026	$ 279,113	$ 216,544	$ 279,113	$ 258,592