Consolidated Statements of Cash Flows - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2017		Dec. 31, 2016	Dec. 31, 2015
Operating Activities
Net income		$ 101,069		$ 278,911	$ 317,459
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization		419,818		845,605	588,238
Equity in net income from unconsolidated joint ventures		(21,892)		(11,874)	(13,028)
Distributions of cumulative earnings from unconsolidated joint ventures		20,309		24,337	40,759
Equity in net gain on sale of interest in unconsolidated joint venture interest/real estate		(16,166)		(44,009)	(15,844)
Purchase price fair value adjustment		0		0	(40,078)
Depreciable real estate reserves		178,520		10,387	19,226
Gain on sale of real estate, net		(73,241)		(238,116)	(175,974)
Gain on sale of discontinued operations		0		0	(14,122)
(Gain) loss on sale of investments in marketable securities		(3,262)		83	0
Loss on early extinguishment of debt		0		0	49
Deferred rents receivable		(38,009)		26,716	(136,924)
Other non-cash adjustments	[1]	17,196		(150,913)	(20,671)
Changes in operating assets and liabilities:
Restricted cash—operations		7,147		(10,811)	11,289
Tenant and other receivables		(5,717)		4,780	(6,405)
Related party receivables		(7,209)		(5,183)	1,278
Deferred lease costs		(41,939)		(70,707)	(61,005)
Other assets		(23,068)		9,899	18,501
Accounts payable, accrued expenses and other liabilities and security deposits		(11,790)		(35,628)	8,634
Deferred revenue and land leases payable		46,607		1,237	5,102
Net cash provided by operating activities		548,373		634,714	526,484
Investing Activities
Acquisitions of real estate property		(13,680)		(37,760)	(2,653,311)
Additions to land, buildings and improvements		(335,253)		(411,950)	(406,442)
Escrowed cash—capital improvements/acquisition deposits/deferred purchase price		(3,499)		92,374	(101,000)
Investments in unconsolidated joint ventures		(389,249)		(84,456)	(161,712)
Distributions in excess of cumulative earnings from unconsolidated joint ventures		319,745		196,211	98,639
Proceeds from disposition of real estate/joint venture interest		639,682		2,469,719	1,216,785
Proceeds from sale of marketable securities		55,129		6,965	1,426
Purchases of marketable securities		0		(43,341)	(7,769)
Other investments		25,330		7,704	(15,806)
Origination of debt and preferred equity investments		(1,129,970)		(977,413)	(756,939)
Repayments or redemption of debt and preferred equity investments		812,914		904,517	520,218
Net cash (used in) provided by investing activities		(18,851)		2,122,570	(2,265,911)
Financing Activities
Proceeds from mortgages and other loans payable		870,459		408,293	1,849,293
Repayments of mortgages and other loans payable		(902,460)		(1,822,303)	(781,236)
Proceeds from revolving credit facility, term loans and senior unsecured notes		2,784,599		1,325,300	2,515,000
Repayments of revolving credit facility, term loans and senior unsecured notes		(2,276,782)		(2,334,604)	(1,706,007)
Payments of debt extinguishment costs		0		0	0
Proceeds from stock options exercised and DRSPP issuance		23,537		15,109	115,917
Proceeds from sale of common stock		0		0	124,761
Repurchase of common stock		(806,302)		0	0
Redemption of preferred stock		(275)		(3,299)	(200)
Distributions to noncontrolling interests in other partnerships		(52,446)		(15,419)	(119,784)
Contributions from noncontrolling interests in other partnerships		36,275		2,359	12,674
Distributions to noncontrolling interests in the Operating Partnership		(14,266)		(12,671)	(9,710)
Dividends/Distributions paid on common and preferred stock/units		(333,543)		(314,079)	(257,378)
Other obligations related to mortgage loan participations		17,227		59,150	25,000
Deferred loan costs and capitalized lease obligation		(27,100)		(41,076)	(54,913)
Net cash (used in) provided by financing activities		(681,077)		(2,733,240)	1,713,417
Net (decrease) increase in cash and cash equivalents		(151,555)		24,044	(26,010)
Cash and cash equivalents at beginning of year		279,443		255,399	281,409
Cash and cash equivalents at end of period		127,888		279,443	255,399
Supplemental Cash Flow Information [Abstract]
Interest paid		273,819		344,295	345,110
Income taxes paid		2,448		2,009	3,882
Supplemental Disclosure of Non-Cash Investing and Financing Activities:
Issuance of units in the operating partnership		25,723		78,495	30,506
Redemption of units in the operating partnership		21,574		31,806	55,697
Derivative instruments at fair value		4,397		31,826	1,816
Exchange of debt investment for equity in joint venture		0		68,581	10,151
Acquisition of subsidiary interest from noncontrolling interest		0		0	20,630
Tenant improvements and capital expenditures payable		6,667		15,972	7,755
Fair value adjustment to noncontrolling interest in operating partnership		5,712		4,222	20,915
Assumption of mortgage loan		0		0	112,795
Capital lease assets		0		0	20,000
Reclassification of development costs from other assets to real estate		0		0	47,519
Deconsolidation of a subsidiary	[2]	695,204		1,226,425	27,435
Transfer of assets to assets held for sale		611,809		2,048,376	34,981
Transfer of liabilities related to assets held for sale		5,364		1,677,528	29,000
Consolidation of real estate investment		0		0	158,566
Removal of fully depreciated commercial real estate properties		15,488		31,474	241,910
Issuance of SLG's common stock to a consolidated joint venture		0		114,049	10,000
Contribution to consolidated joint venture by noncontrolling interest		0		0	22,504
Share repurchase payable		41,746		0	0
SL Green Operating Partnership
Operating Activities
Net income		101,069		278,911	317,459
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization		419,818		845,605	588,238
Equity in net income from unconsolidated joint ventures		(21,892)		(11,874)	(13,028)
Distributions of cumulative earnings from unconsolidated joint ventures		20,309		24,337	40,759
Equity in net gain on sale of interest in unconsolidated joint venture interest/real estate		(16,166)		(44,009)	(15,844)
Purchase price fair value adjustment		0		0	(40,078)
Depreciable real estate reserves		178,520		10,387	19,226
Gain on sale of real estate, net		(73,241)		(238,116)	(175,974)
Gain on sale of discontinued operations		0		0	(14,122)
(Gain) loss on sale of investments in marketable securities		(3,262)		83	0
Loss on early extinguishment of debt		0		0	49
Deferred rents receivable		(38,009)		26,716	(136,924)
Other non-cash adjustments		17,196	[1]	(150,913)	(20,671)
Changes in operating assets and liabilities:
Restricted cash—operations		7,147		(10,811)	11,289
Tenant and other receivables		(5,717)		4,780	(6,405)
Related party receivables		(7,209)		(5,183)	1,278
Deferred lease costs		(41,939)		(70,707)	(61,005)
Other assets		(23,068)		9,899	18,501
Accounts payable, accrued expenses and other liabilities and security deposits		(11,790)		(35,628)	8,634
Deferred revenue and land leases payable		46,607		1,237	5,102
Net cash provided by operating activities		548,373		634,714	526,484
Investing Activities
Acquisitions of real estate property		(13,680)		(37,760)	(2,653,311)
Additions to land, buildings and improvements		(335,253)		(411,950)	(406,442)
Escrowed cash—capital improvements/acquisition deposits/deferred purchase price		(3,499)		92,374	(101,000)
Investments in unconsolidated joint ventures		(389,249)		(84,456)	(161,712)
Distributions in excess of cumulative earnings from unconsolidated joint ventures		319,745		196,211	98,639
Proceeds from disposition of real estate/joint venture interest		639,682		2,469,719	1,216,785
Proceeds from sale of marketable securities		55,129		6,965	1,426
Purchases of marketable securities		0		(43,341)	(7,769)
Other investments		25,330		7,704	(15,806)
Origination of debt and preferred equity investments		(1,129,970)		(977,413)	(756,939)
Repayments or redemption of debt and preferred equity investments		812,914		904,517	520,218
Net cash (used in) provided by investing activities		(18,851)		2,122,570	(2,265,911)
Financing Activities
Proceeds from mortgages and other loans payable		870,459		408,293	1,849,293
Repayments of mortgages and other loans payable		(902,460)		(1,822,303)	(781,236)
Proceeds from revolving credit facility, term loans and senior unsecured notes		2,784,599		1,325,300	2,515,000
Repayments of revolving credit facility, term loans and senior unsecured notes		(2,276,782)		(2,334,604)	(1,706,007)
Payments of debt extinguishment costs		0		0	0
Proceeds from stock options exercised and DRSPP issuance		23,537		15,109	115,917
Proceeds from sale of common stock		0		0	124,761
Repurchase of common stock		(806,302)		0	0
Redemption of preferred stock		(275)		(3,299)	(200)
Distributions to noncontrolling interests in other partnerships		(52,446)		(15,419)	(119,784)
Contributions from noncontrolling interests in other partnerships		36,275		2,359	12,674
Dividends/Distributions paid on common and preferred stock/units		(347,809)		(326,750)	(267,088)
Other obligations related to mortgage loan participations		17,227		59,150	25,000
Deferred loan costs and capitalized lease obligation		(27,100)		(41,076)	(54,913)
Net cash (used in) provided by financing activities		(681,077)		(2,733,240)	1,713,417
Net (decrease) increase in cash and cash equivalents		(151,555)		24,044	(26,010)
Cash and cash equivalents at beginning of year		279,443		255,399	281,409
Cash and cash equivalents at end of period		127,888		279,443	255,399
Supplemental Cash Flow Information [Abstract]
Interest paid		273,819		344,295	345,110
Income taxes paid		2,448		2,009	3,882
Supplemental Disclosure of Non-Cash Investing and Financing Activities:
Issuance of units in the operating partnership		25,723		78,495	30,506
Derivative instruments at fair value		4,397		31,826	1,816
Exchange of debt investment for equity in joint venture		0		68,581	10,151
Acquisition of subsidiary interest from noncontrolling interest		0		0	20,630
Tenant improvements and capital expenditures payable		6,667		15,972	7,755
Fair value adjustment to noncontrolling interest in operating partnership		5,712		4,222	20,915
Assumption of mortgage loan		0		0	112,795
Capital lease assets		0		0	20,000
Reclassification of development costs from other assets to real estate		0		0	47,519
Deconsolidation of a subsidiary	[2]	695,204		1,226,425	27,435
Transfer of assets to assets held for sale		611,809		2,048,376	34,981
Transfer of liabilities related to assets held for sale		5,364		1,677,528	29,000
Consolidation of real estate investment		0		0	158,566
Removal of fully depreciated commercial real estate properties		15,488		31,474	241,910
Issuance of SLG's common stock to a consolidated joint venture		0		114,049	10,000
Contribution to consolidated joint venture by noncontrolling interest		0		0	22,504
Share repurchase payable		41,746		0	0
Common Stock
Supplemental Disclosure of Non-Cash Investing and Financing Activities:
Issuance of stock/units relating to the real estate acquisition		0		0	2,228
Common Stock | 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	2,228
Preferred Units
Supplemental Disclosure of Non-Cash Investing and Financing Activities:
Issuance of stock/units relating to the real estate acquisition		0		22,793	211,601
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		$ 22,793	$ 211,601

[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.".