Consolidated Statement of Capital shares in Thousands, $ in Thousands		12 Months Ended
		Dec. 31, 2020 USD ($) shares	Dec. 31, 2019 USD ($) shares	Dec. 31, 2018 USD ($) shares	Dec. 31, 2017 USD ($) shares
Increase (Decrease) in Partner's Capital
Accounting Standards Update [Extensible List]			us-gaap:AccountingStandardsUpdate201613Member		us-gaap:AccountingStandardsUpdate201705Member
Acquisition of subsidiary interest from noncontrolling interest		$ (1,536)	$ (25,845)
DRSPP proceeds		1,006	334	$ 136
Conversion of common units		8,744	471	16,303
Reallocation of noncontrolling interest in the Operating Partnership		32,598	(34,320)	34,236
Deferred compensation plan and stock awards, net of forfeitures and tax withholdings		25,271	25,763	17,484
Repurchases of common units		(532,262)	(384,399)	(937,795)
Deconsolidation of partially owned entity				(315,116)
Distributions to noncontrolling interests		$ (78,855)	$ (478)	$ (8,364)
Common Stock
Increase (Decrease) in Partner's Capital
DRSPP proceeds (in units) | shares	[1]	17	4	1
DRSPP proceeds
Conversion of common units		$ 1		$ 2
Deferred compensation plan and stock awards, net of forfeitures and tax withholdings (in units) | shares	[1]	(34)	102	145
Deferred compensation plan and stock awards, net of forfeitures and tax withholdings		$ 0	$ 2	$ 1
Repurchases of common units (in units) | shares	[1]	(8,529)	(4,466)	(9,469)
Repurchases of common units		$ (88)	$ (46)	$ (98)
Noncontrolling Interests
Increase (Decrease) in Partner's Capital
Acquisition of subsidiary interest from noncontrolling interest		1,587	(25,276)
Deconsolidation of partially owned entity				(315,116)
Distributions to noncontrolling interests		(78,855)	(478)	(8,364)
SL Green Operating Partnership
Increase (Decrease) in Partner's Capital
Beginning Balance		5,517,198	5,947,855	6,589,454
Net income (loss)		385,995	267,275	247,256
Acquisition of subsidiary interest from noncontrolling interest		(1,536)	(25,845)
Other comprehensive income (loss)		(38,762)	(43,593)	(3,496)
Preferred dividends		(14,950)	(14,950)	(14,950)
DRSPP proceeds		1,006	334	136
Conversion of common units		8,744	471	16,303
Reallocation of noncontrolling interest in the Operating Partnership		32,598	(34,320)	34,236
Deferred compensation plan and stock awards, net of forfeitures and tax withholdings		25,271	25,763	17,484
Repurchases of common units		(532,262)	(384,399)	(937,795)
Proceeds from stock options exercised				28,912
Contributions to consolidated joint venture interests		12,477	58,462	5,459
Distributions to noncontrolling interests		(78,855)	(478)	(8,364)
Cash distributions declared (none of which represented a return of capital for federal income tax purposes)		(341,945)	(279,377)	(282,188)
Ending Balance		4,935,795	5,517,198	5,947,855	$ 6,589,454
SL Green Operating Partnership | Cumulative Effect, Period of Adoption, Adjustment
Increase (Decrease) in Partner's Capital
Beginning Balance		(39,184)		570,524
Ending Balance			(39,184)		570,524
SL Green Operating Partnership | Cumulative Effect, Period of Adoption, Adjusted Balance
Increase (Decrease) in Partner's Capital
Beginning Balance		5,478,014		7,159,978
Ending Balance			5,478,014		7,159,978
SL Green Operating Partnership | Preferred Units | Series I Preferred Stock
Increase (Decrease) in Partner's Capital
Beginning Balance		221,932	221,932	221,932
Ending Balance		221,932	221,932	221,932	221,932
SL Green Operating Partnership | Preferred Units | Series I Preferred Stock | Cumulative Effect, Period of Adoption, Adjusted Balance
Increase (Decrease) in Partner's Capital
Beginning Balance		221,932		221,932
Ending Balance			221,932		221,932
SL Green Operating Partnership | Common Stock | Partners' Interest
Increase (Decrease) in Partner's Capital
Beginning Balance		$ 5,247,868	$ 5,664,481	$ 5,984,557
Beginning Balance (units) | shares	[2]	76,956	81,311	90,172
Net income (loss)		$ 371,055	$ 270,434	$ 247,262
Acquisition of subsidiary interest from noncontrolling interest		(3,123)	(569)
Preferred dividends		$ (14,950)	$ (14,950)	$ (14,950)
DRSPP proceeds (in units) | shares	[2]	17	4	1
DRSPP proceeds		$ 1,006	$ 334	$ 136
Conversion of common units (in units) | shares	[2]	98	5	155
Conversion of common units		$ 8,744	$ 471	$ 16,303
Reallocation of noncontrolling interest in the Operating Partnership		$ 32,598	$ (34,320)	$ 34,236
Deferred compensation plan and stock awards, net of forfeitures and tax withholdings (in units) | shares	[2]	(34)	102	145
Deferred compensation plan and stock awards, net of forfeitures and tax withholdings		$ 25,271	$ 25,763	$ 17,484
Repurchases of common units (in units) | shares	[2]	(8,529)	(4,466)	(9,469)
Repurchases of common units		$ (532,262)	$ (384,399)	$ (937,795)
Proceeds from stock options exercised (in units) | shares	[2]			307
Proceeds from stock options exercised				$ 28,912
Cash distributions declared (none of which represented a return of capital for federal income tax purposes)		(341,945)	(279,377)	(282,188)
Ending Balance		$ 4,755,078	$ 5,247,868	$ 5,664,481	$ 5,984,557
Ending Balance (units) | shares	[2]	68,508	76,956	81,311	90,172
SL Green Operating Partnership | Common Stock | Partners' Interest | Cumulative Effect, Period of Adoption, Adjustment
Increase (Decrease) in Partner's Capital
Beginning Balance		$ (39,184)		$ 570,524
Ending Balance			$ (39,184)		$ 570,524
SL Green Operating Partnership | Common Stock | Partners' Interest | Cumulative Effect, Period of Adoption, Adjusted Balance
Increase (Decrease) in Partner's Capital
Beginning Balance		$ 5,208,684		$ 6,555,081
Beginning Balance (units) | shares	[2]	76,956		90,172
Ending Balance			$ 5,208,684		$ 6,555,081
Ending Balance (units) | shares	[2]		76,956		90,172
SL Green Operating Partnership | Accumulated Other Comprehensive Income (Loss)
Increase (Decrease) in Partner's Capital
Beginning Balance		$ (28,485)	$ 15,108	$ 18,604
Other comprehensive income (loss)		(38,762)	(43,593)	(3,496)
Ending Balance		(67,247)	(28,485)	15,108	$ 18,604
SL Green Operating Partnership | Accumulated Other Comprehensive Income (Loss) | Cumulative Effect, Period of Adoption, Adjusted Balance
Increase (Decrease) in Partner's Capital
Beginning Balance		(28,485)		18,604
Ending Balance			(28,485)		18,604
SL Green Operating Partnership | Noncontrolling Interests
Increase (Decrease) in Partner's Capital
Beginning Balance		75,883	46,334	364,361
Net income (loss)		14,940	(3,159)	(6)
Acquisition of subsidiary interest from noncontrolling interest		1,587	(25,276)
Contributions to consolidated joint venture interests		12,477	58,462	5,459
Deconsolidation of partially owned entity				(315,116)
Distributions to noncontrolling interests		(78,855)	(478)	(8,364)
Ending Balance		26,032	75,883	46,334	364,361
SL Green Operating Partnership | Noncontrolling Interests | Cumulative Effect, Period of Adoption, Adjusted Balance
Increase (Decrease) in Partner's Capital
Beginning Balance		$ 75,883		$ 364,361
Ending Balance			$ 75,883		$ 364,361

[1] On January 21, 2021, we completed a reverse stock split whereby every 1.02918 SL Green common share was combined into 1 SL Green common share. We have retroactively adjusted the outstanding share counts, share activity, cash distributions declared, and earnings per share, as if the reverse split occurred on December 31, 2017.

[2] On January 21, 2021, we completed a reverse stock split whereby every 1.02918 SL Green Operating Partnership common unit was combined into 1 SL Green Operating Partnership common unit. We have retroactively adjusted the outstanding unit counts, unit activity, cash distributions declared, and earnings per units, as if the reverse split occurred on December 31, 2017