Consolidated Statements of Cash Flows - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2018		Dec. 31, 2017	Dec. 31, 2016
Operating Activities
Net income		$ 270,856		$ 101,069	$ 278,911
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization		291,915		419,818	845,605
Equity in net income from unconsolidated joint ventures		(7,311)		(21,892)	(11,874)
Distributions of cumulative earnings from unconsolidated joint ventures		10,277		20,309	24,337
Equity in net gain on sale of interest in unconsolidated joint venture interest/real estate		(303,967)		(16,166)	(44,009)
Purchase price and other fair value adjustment		(57,385)		0	0
Depreciable real estate reserves and impairment		227,543		178,520	10,387
Loss (gain) on sale of real estate, net		30,757		(73,241)	(238,116)
Loan loss reserves and other investment reserves, net of recoveries		6,839		0	0
(Gain) loss on sale of investments in marketable securities		0		(3,262)	83
Loss on early extinguishment of debt		17,083		0	0
Deferred rents receivable		(18,216)		(38,009)	26,716
Other non-cash adjustments	[1]	2,932		19,621	(152,428)
Changes in operating assets and liabilities:
Tenant and other receivables		6,968		(5,717)	4,780
Related party receivables		(1,044)		(7,209)	(5,183)
Deferred lease costs		(44,158)		(41,939)	(70,707)
Other assets		(8,310)		(23,068)	9,899
Accounts payable, accrued expenses and other liabilities and security deposits		4,410		(12,440)	(35,628)
Deferred revenue and land leases payable		12,348		46,607	1,237
Net cash provided by operating activities		441,537		543,001	644,010
Investing Activities
Acquisitions of real estate property		(60,486)		(28,680)	(39,890)
Additions to land, buildings and improvements		(254,460)		(336,001)	(411,950)
Investments in unconsolidated joint ventures		(400,429)		(389,249)	(145,375)
Distributions in excess of cumulative earnings from unconsolidated joint ventures		233,118		319,745	196,211
Proceeds from disposition of real estate/joint venture interest		1,231,004		692,796	2,475,954
Proceeds from sale of marketable securities		0		55,129	6,965
Purchases of marketable securities		0		0	(43,341)
Other investments		(38,912)		25,330	7,704
Origination of debt and preferred equity investments		(731,216)		(1,129,970)	(977,413)
Repayments or redemption of debt and preferred equity investments		703,043		812,914	904,517
Net cash provided by investing activities		681,662		22,014	1,973,382
Financing Activities
Proceeds from mortgages and other loans payable		564,391		870,459	408,293
Repayments of mortgages and other loans payable		(868,842)		(902,460)	(1,822,303)
Proceeds from revolving credit facility, term loans and senior unsecured notes		3,120,000		2,784,599	1,325,300
Repayments of revolving credit facility, term loans and senior unsecured notes		(2,560,000)		(2,276,782)	(2,334,604)
Payments of debt extinguishment costs		13,918		0	0
Proceeds from stock options exercised and DRSPP issuance		29,048		23,537	15,109
Repurchase of common stock		(979,541)		(806,302)	0
Redemption of preferred stock		(1,208)		(275)	(3,299)
Redemption of OP units		(33,972)		0	0
Distributions to noncontrolling interests in other partnerships		(8,364)		(52,446)	(15,419)
Contributions from noncontrolling interests in other partnerships		5,459		36,275	2,359
Distributions to noncontrolling interests in the Operating Partnership		(15,000)		(14,266)	(12,671)
Dividends/Distributions paid on common and preferred stock/units		(313,230)		(333,543)	(314,079)
Other obligations related to mortgage loan participations		16		17,227	59,150
Payment of tax witholdings for restricted share awards		(3,842)		(3,879)	(3,162)
Deferred loan costs and capitalized lease obligation		(15,109)		(27,100)	(41,076)
Net cash used in by financing activities		(1,094,112)		(684,956)	(2,736,402)
Net increase (decrease) in cash and cash equivalents		29,087		(119,941)	(119,010)
Cash, restricted cash, and cash equivalents at beginning of year		250,026		369,967
Cash, restricted cash, and cash equivalents at end of period		279,113		250,026	369,967
Supplemental Cash Flow Information [Abstract]
Interest paid		259,776		273,819	344,295
Income taxes paid		1,418		2,448	2,009
Supplemental Disclosure of Non-Cash Investing and Financing Activities:
Issuance of units in the operating partnership		0		25,723	78,495
Redemption of units in the operating partnership		16,303		21,574	31,806
Redemption of units in the operating partnership for a joint venture sale		10,445		0	0
Exchange of debt investment for real estate or equity in joint venture		298,956		0	68,581
Tenant improvements and capital expenditures payable		0		6,667	15,972
Fair value adjustment to noncontrolling interest in operating partnership		34,236		5,712	4,222
Deconsolidation of a subsidiary	[2]	298,404		695,204	1,226,425
Transfer of assets to assets held for sale		0		611,809	2,048,376
Transfer of liabilities related to assets held for sale		0		5,364	1,677,528
Removal of fully depreciated commercial real estate properties		124,249		15,488	31,474
Issuance of SLG's common stock to a consolidated joint venture		0		0	114,049
Share repurchase payable		0		41,746	0
SL Green Operating Partnership
Operating Activities
Net income		270,856		101,069	278,911
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization		291,915		419,818	845,605
Equity in net income from unconsolidated joint ventures		(7,311)		(21,892)	(11,874)
Distributions of cumulative earnings from unconsolidated joint ventures		10,277		20,309	24,337
Equity in net gain on sale of interest in unconsolidated joint venture interest/real estate		(303,967)		(16,166)	(44,009)
Purchase price and other fair value adjustment		(57,385)		0	0
Depreciable real estate reserves and impairment		227,543		178,520	10,387
Loss (gain) on sale of real estate, net		30,757		(73,241)	(238,116)
Loan loss reserves and other investment reserves, net of recoveries		6,839		0	0
(Gain) loss on sale of investments in marketable securities		0		(3,262)	83
Loss on early extinguishment of debt		17,083		0	0
Deferred rents receivable		(18,216)		(38,009)	26,716
Other non-cash adjustments		2,932	[1]	19,621	(152,428)
Changes in operating assets and liabilities:
Tenant and other receivables		6,968		(5,717)	4,780
Related party receivables		(1,044)		(7,209)	(5,183)
Deferred lease costs		(44,158)		(41,939)	(70,707)
Other assets		(8,310)		(23,068)	9,899
Accounts payable, accrued expenses and other liabilities and security deposits		4,410		(12,440)	(35,628)
Deferred revenue and land leases payable		12,348		46,607	1,237
Net cash provided by operating activities		441,537		543,001	644,010
Investing Activities
Acquisitions of real estate property		(60,486)		(28,680)	(39,890)
Additions to land, buildings and improvements		(254,460)		(336,001)	(411,950)
Investments in unconsolidated joint ventures		(400,429)		(389,249)	(145,375)
Distributions in excess of cumulative earnings from unconsolidated joint ventures		233,118		319,745	196,211
Proceeds from disposition of real estate/joint venture interest		1,231,004		692,796	2,475,954
Proceeds from sale of marketable securities		0		55,129	6,965
Purchases of marketable securities		0		0	(43,341)
Other investments		(38,912)		25,330	7,704
Origination of debt and preferred equity investments		(731,216)		(1,129,970)	(977,413)
Repayments or redemption of debt and preferred equity investments		703,043		812,914	904,517
Net cash provided by investing activities		681,662		22,014	1,973,382
Financing Activities
Proceeds from mortgages and other loans payable		564,391		870,459	408,293
Repayments of mortgages and other loans payable		(868,842)		(902,460)	(1,822,303)
Proceeds from revolving credit facility, term loans and senior unsecured notes		3,120,000		2,784,599	1,325,300
Repayments of revolving credit facility, term loans and senior unsecured notes		(2,560,000)		(2,276,782)	(2,334,604)
Payments of debt extinguishment costs		13,918		0	0
Proceeds from stock options exercised and DRSPP issuance		29,048		23,537	15,109
Repurchase of common stock		(979,541)		(806,302)	0
Redemption of preferred stock		(1,208)		(275)	(3,299)
Redemption of OP units		(33,972)		0	0
Distributions to noncontrolling interests in other partnerships		(8,364)		(52,446)	(15,419)
Contributions from noncontrolling interests in other partnerships		5,459		36,275	2,359
Dividends/Distributions paid on common and preferred stock/units		(328,230)		(347,809)	(326,750)
Other obligations related to mortgage loan participations		16		17,227	59,150
Payment of tax witholdings for restricted share awards		(3,842)		(3,879)	(3,162)
Deferred loan costs and capitalized lease obligation		(15,109)		(27,100)	(41,076)
Net cash used in by financing activities		(1,094,112)		(684,956)	(2,736,402)
Net increase (decrease) in cash and cash equivalents		29,087		(119,941)	(119,010)
Cash, restricted cash, and cash equivalents at beginning of year		250,026		369,967	488,977
Cash, restricted cash, and cash equivalents at end of period		279,113		250,026	369,967
Supplemental Cash Flow Information [Abstract]
Interest paid		259,776		273,819	344,295
Income taxes paid		1,418		2,448	2,009
Supplemental Disclosure of Non-Cash Investing and Financing Activities:
Issuance of units in the operating partnership		0		25,723	78,495
Redemption of units in the operating partnership		16,303		21,574	31,806
Redemption of units in the operating partnership for a joint venture sale		10,445		0	0
Exchange of debt investment for real estate or equity in joint venture		298,956		0	68,581
Tenant improvements and capital expenditures payable		0		6,667	15,972
Fair value adjustment to noncontrolling interest in operating partnership		34,236		5,712	4,222
Deconsolidation of a subsidiary	[2]	298,404		695,204	1,226,425
Transfer of assets to assets held for sale		0		611,809	2,048,376
Transfer of liabilities related to assets held for sale		0		5,364	1,677,528
Removal of fully depreciated commercial real estate properties		124,249		15,488	31,474
Issuance of SLG's common stock to a consolidated joint venture		0		0	114,049
Share repurchase payable		0		41,746	0
Preferred Units
Supplemental Disclosure of Non-Cash Investing and Financing Activities:
Issuance of stock/units relating to the real estate acquisition		0		0	22,793
Preferred Units | SL Green Operating Partnership
Supplemental Disclosure of Non-Cash Investing and Financing Activities:
Issuance of stock/units relating to the real estate acquisition		$ 0		$ 0	$ 22,793

[1] Included in Other non-cash adjustments is $172.4 million for the year ended December 31, 2016 for the amortization of the below-market lease at 388-390 Greenwich Street as a result of the tenant exercising their option to purchase the property and entering into an agreement to accelerate the sale.

[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. See Note 6, "Investments in Unconsolidated Joint Ventures.".