Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Operating Activities
Net income	$ 414,758	$ 291,487	$ 270,856
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	325,462	284,011	289,899
Equity in net loss (income) from unconsolidated joint ventures	25,195	34,518	(7,311)
Distributions of cumulative earnings from unconsolidated joint ventures	679	864	10,277
Equity in net gain on sale of interest in unconsolidated joint venture interest/real estate	(2,961)	(76,181)	(303,967)
Purchase price and other fair value adjustments	(187,522)	(69,389)	(57,385)
Depreciable real estate reserves and impairments	60,454	7,047	227,543
(Gain) loss on sale of real estate, net	(215,506)	16,749	30,757
Loan loss reserves and other investment reserves, net of recoveries	35,298	0	6,839
Loss on early extinguishment of debt	0	0	17,083
Deferred rents receivable	(7,582)	(13,941)	(18,216)
Non-cash lease expense	11,984	13,744	2,016
Other non-cash adjustments	15,178	271	2,932
Changes in operating assets and liabilities:
Tenant and other receivables	(17,074)	(4,968)	6,968
Related party receivables	1,451	7,802	(1,044)
Deferred lease costs	(20,900)	(70,938)	(44,158)
Other assets	(26,137)	(18,630)	(8,310)
Accounts payable, accrued expenses, other liabilities and security deposits	132,171	(25,597)	4,410
Deferred revenue	20,657	10,824	12,348
Change in lease liability - operating leases	(11,369)	(11,200)	0
Net cash provided by operating activities	554,236	376,473	441,537
Investing Activities
Acquisitions of real estate property	(86,846)	(262,591)	(60,486)
Additions to land, buildings and improvements	(458,140)	(252,986)	(254,460)
Acquisition deposits and deferred purchase price	0	(5,239)	0
Investments in unconsolidated joint ventures	(70,315)	(128,682)	(400,429)
Distributions in excess of cumulative earnings from unconsolidated joint ventures	124,572	79,020	233,118
Net proceeds from disposition of real estate/joint venture interest	1,112,382	208,302	1,231,004
Other investments	32,479	(7,869)	(38,912)
Origination of debt and preferred equity investments	(360,953)	(607,844)	(731,216)
Repayments or redemption of debt and preferred equity investments	763,251	1,092,383	703,043
Net cash provided by investing activities	1,056,430	114,494	681,662
Financing Activities
Proceeds from mortgages and other loans payable	1,181,892	752,984	564,391
Repayments of mortgages and other loans payable	(1,186,828)	(230,076)	(868,842)
Proceeds from revolving credit facility and senior unsecured notes	1,495,000	1,310,000	3,120,000
Repayments of revolving credit facility and senior unsecured notes	(1,875,000)	(1,570,000)	(2,560,000)
Payment of debt extinguishment costs	0	0	(13,918)
Proceeds from stock options exercised and DRSPP issuance	1,006	334	29,048
Repurchase of common stock	(528,483)	(384,399)	(979,541)
Redemption of preferred stock	(82,750)	(18,142)	(1,208)
Redemption of OP units	(27,342)	(27,495)	(33,972)
Distributions to noncontrolling interests in other partnerships	(85,468)	(478)	(8,364)
Contributions from noncontrolling interests in other partnerships	12,477	10,239	5,459
Acquisition of subsidiary interest from noncontrolling interest	(1,536)	(25,845)	0
Distributions to noncontrolling interests in the Operating Partnership	(12,652)	(14,729)	(15,000)
Dividends/Distributions paid on common and preferred stock/units	(293,996)	(306,386)	(313,230)
Other obligations related to loan participations	0	0	16
Tax withholdings related to restricted share awards	(4,752)	(3,495)	(3,842)
Deferred loan costs	(70,036)	(21,162)	(15,109)
Principal payments of on financing lease liabilities	(833)	0	0
Net cash used in financing activities	(1,479,301)	(528,650)	(1,094,112)
Net increase (decrease) in cash, cash equivalents, and restricted cash	131,365	(37,683)	29,087
Cash, cash equivalents, and restricted cash at beginning of year	241,430	279,113	250,026
Cash, cash equivalents, and restricted cash at end of period	372,795	241,430	279,113
Supplemental Cash Flow Information [Abstract]
Interest paid	201,348	248,684	259,776
Income taxes paid	2,296	1,489	1,418
Supplemental Disclosure of Non-Cash Investing and Financing Activities:
Conversion of units in the Operating Partnership	8,744	471	16,303
Redemption of units in the Operating Partnership for a joint venture sale	0	0	10,445
Exchange of preferred equity investment for real estate or equity in joint venture	119,497	0	0
Exchange of debt investment for real estate or equity in joint venture	122,796	34,498	298,956
Tenant improvements and capital expenditures payable	1,665	6,056	0
Fair value adjustment to noncontrolling interest in the Operating Partnership	32,598	34,320	34,236
Deconsolidation of a subsidiary	854,437	395	298,404
Deconsolidation of a subsidiary mortgage	5,593	0	0
Mortgages assumed in connection with sale of real estate	250,000	0	0
Seller financed purchases	100,000	0	0
Debt and preferred equity investments	9,014	0	0
Transfer of assets related to assets held for sale	0	391,664	0
Reversal of assets held for sale	391,664	0	0
Transfer of liabilities related to assets held for sale	0	0	0
Removal of fully depreciated commercial real estate properties	66,169	19,577	124,249
Contribution to consolidated joint venture by noncontrolling interest	0	48,223	0
Share repurchase payable	3,779	0	0
Recognition of sales-type leases and related lease liabilities	119,725	0	0
Recognition of right of use assets and related lease liabilities	61,990	389,120	0
Total cash, cash equivalents, and restricted cash	241,430	241,430	250,026
SL Green Operating Partnership
Operating Activities
Net income	414,758	291,487	270,856
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	325,462	284,011	289,899
Equity in net loss (income) from unconsolidated joint ventures	25,195	34,518	(7,311)
Distributions of cumulative earnings from unconsolidated joint ventures	679	864	10,277
Equity in net gain on sale of interest in unconsolidated joint venture interest/real estate	(2,961)	(76,181)	(303,967)
Purchase price and other fair value adjustments	(187,522)	(69,389)	(57,385)
Depreciable real estate reserves and impairments	60,454	7,047	227,543
(Gain) loss on sale of real estate, net	(215,506)	16,749	30,757
Loan loss reserves and other investment reserves, net of recoveries	35,298	0	6,839
Loss on early extinguishment of debt	0	0	17,083
Deferred rents receivable	(7,582)	(13,941)	(18,216)
Non-cash lease expense	11,984	13,744	2,016
Other non-cash adjustments	15,178	271	2,932
Changes in operating assets and liabilities:
Tenant and other receivables	(17,074)	(4,968)	6,968
Related party receivables	1,451	7,802	(1,044)
Deferred lease costs	(20,900)	(70,938)	(44,158)
Other assets	(26,137)	(18,630)	(8,310)
Accounts payable, accrued expenses, other liabilities and security deposits	132,171	(25,597)	4,410
Deferred revenue	20,657	10,824	12,348
Change in lease liability - operating leases	(11,369)	(11,200)	0
Net cash provided by operating activities	554,236	376,473	441,537
Investing Activities
Acquisitions of real estate property	(86,846)	(262,591)	(60,486)
Additions to land, buildings and improvements	(458,140)	(252,986)	(254,460)
Acquisition deposits and deferred purchase price	0	(5,239)	0
Investments in unconsolidated joint ventures	(70,315)	(128,682)	(400,429)
Distributions in excess of cumulative earnings from unconsolidated joint ventures	124,572	79,020	233,118
Net proceeds from disposition of real estate/joint venture interest	1,112,382	208,302	1,231,004
Other investments	32,479	(7,869)	(38,912)
Origination of debt and preferred equity investments	(360,953)	(607,844)	(731,216)
Repayments or redemption of debt and preferred equity investments	763,251	1,092,383	703,043
Net cash provided by investing activities	1,056,430	114,494	681,662
Financing Activities
Proceeds from mortgages and other loans payable	1,181,892	752,984	564,391
Repayments of mortgages and other loans payable	(1,186,828)	(230,076)	(868,842)
Proceeds from revolving credit facility and senior unsecured notes	1,495,000	1,310,000	3,120,000
Repayments of revolving credit facility and senior unsecured notes	(1,875,000)	(1,570,000)	(2,560,000)
Payment of debt extinguishment costs	0	0	(13,918)
Proceeds from stock options exercised and DRSPP issuance	1,006	334	29,048
Repurchase of common stock	(528,483)	(384,399)	(979,541)
Redemption of preferred stock	(82,750)	(18,142)	(1,208)
Redemption of OP units	(27,342)	(27,495)	(33,972)
Distributions to noncontrolling interests in other partnerships	(85,468)	(478)	(8,364)
Contributions from noncontrolling interests in other partnerships	12,477	10,239	5,459
Acquisition of subsidiary interest from noncontrolling interest	(1,536)	(25,845)	0
Dividends/Distributions paid on common and preferred stock/units	(306,648)	(321,115)	(328,230)
Other obligations related to loan participations	0	0	16
Tax withholdings related to restricted share awards	(4,752)	(3,495)	(3,842)
Deferred loan costs	(70,036)	(21,162)	(15,109)
Principal payments of on financing lease liabilities	(833)	0	0
Net cash used in financing activities	(1,479,301)	(528,650)	(1,094,112)
Net increase (decrease) in cash, cash equivalents, and restricted cash	131,365	(37,683)	29,087
Cash, cash equivalents, and restricted cash at beginning of year	241,430	279,113	250,026
Cash, cash equivalents, and restricted cash at end of period	372,795	241,430	279,113
Supplemental Cash Flow Information [Abstract]
Interest paid	201,348	248,684	259,776
Income taxes paid	2,296	1,489	1,418
Supplemental Disclosure of Non-Cash Investing and Financing Activities:
Conversion of units in the Operating Partnership	8,744	471	16,303
Redemption of units in the Operating Partnership for a joint venture sale	0	0	10,445
Exchange of preferred equity investment for real estate or equity in joint venture	119,497	0	0
Exchange of debt investment for real estate or equity in joint venture	122,796	34,498	298,956
Tenant improvements and capital expenditures payable	1,665	6,056	0
Fair value adjustment to noncontrolling interest in the Operating Partnership	32,598	34,320	34,236
Deconsolidation of a subsidiary	854,437	395	298,404
Deconsolidation of a subsidiary mortgage	5,593	0	0
Mortgages assumed in connection with sale of real estate	250,000	0	0
Seller financed purchases	100,000	0	0
Debt and preferred equity investments	9,014	0	0
Transfer of assets related to assets held for sale	0	391,664	0
Reversal of assets held for sale	391,664	0	0
Transfer of liabilities related to assets held for sale	0	0	0
Removal of fully depreciated commercial real estate properties	66,169	19,577	124,249
Contribution to consolidated joint venture by noncontrolling interest	0	48,223	0
Share repurchase payable	3,779	0	0
Recognition of sales-type leases and related lease liabilities	119,725	0	0
Recognition of right of use assets and related lease liabilities	61,990	389,120	0
Total cash, cash equivalents, and restricted cash	372,795	279,113	250,026
Preferred Units
Supplemental Disclosure of Non-Cash Investing and Financing Activities:
Issuance of preferred units relating to a real estate acquisition	0	1,000	0
Preferred Units | SL Green Operating Partnership
Supplemental Disclosure of Non-Cash Investing and Financing Activities:
Issuance of preferred units relating to a real estate acquisition	$ 0	$ 1,000	$ 0