Consolidated Statements of Capital - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020	Dec. 31, 2019
Increase (Decrease) in Partner's Capital
Accounting Standards Update		us-gaap:AccountingStandardsUpdate201613Member		us-gaap:AccountingStandardsUpdate201613Member
Acquisition of subsidiary interest from noncontrolling interest		$ (1,536)
DRSPP proceeds	$ 351	166
Conversion of common units		84
Reallocation of noncontrolling interests in the operating partnership	(26,609)	38,529
Deferred compensation plan and stock awards, net of forfeitures and tax withholdings	6,728	5,502
Repurchases of common stock	(80,310)	(219,576)
Contribution to consolidated joint venture interests	171	3,814
Cash distributions to noncontrolling interests	$ (110)	$ (492)
Cash distribution declared, per common share (in dollars per share)	$ 0.910	$ 0.304
Common Stock
Increase (Decrease) in Partner's Capital
DRSPP proceeds (in shares)	5	2
Conversion of common units
Deferred compensation plan and stock awards, net of forfeitures and tax withholdings (in shares)	113	(32)
Deferred compensation plan and stock awards, net of forfeitures and tax withholdings	$ 2	$ (1)
Repurchases of common stock (in shares)	(1,306)	(2,562)
Repurchases of common stock	$ (13)	$ (26)
Noncontrolling Interests
Increase (Decrease) in Partner's Capital
Acquisition of subsidiary interest from noncontrolling interest		1,587
Contribution to consolidated joint venture interests	171	3,814
Cash distributions to noncontrolling interests	(110)	(492)
SL Green Operating Partnership
Increase (Decrease) in Partner's Capital
Beginning Balance	4,935,795	5,517,198	$ 5,517,198
Net loss	(5,225)	118,246
Acquisition of subsidiary interest from noncontrolling interest		(1,536)
Other comprehensive income	48,350	(52,383)
Preferred distributions	(3,738)	(3,738)
DRSPP proceeds	351	166
Conversion of common units		84
Reallocation of noncontrolling interests in the operating partnership	(26,609)	38,529
Deferred compensation plan and stock awards, net of forfeitures and tax withholdings	6,728	5,502
Repurchases of common stock	(80,310)	(219,576)
Contribution to consolidated joint venture interests	171	3,814
Cash distributions to noncontrolling interests	(110)	$ (492)
Issuance of special distribution paid primarily in units	$ 123,529
Cash distribution declared, per common share (in dollars per share)	$ 0.910	$ 0.304
Cash distributions declared (per common unit, none of which represented a return of capital for federal income tax purposes)	$ (63,312)	$ (22,665)
Ending Balance	4,935,620	5,343,965	4,935,795	$ 5,517,198
SL Green Operating Partnership | Common Stock | Partners' Interest
Increase (Decrease) in Partner's Capital
Beginning Balance	$ 4,755,078	$ 5,247,868	$ 5,247,868
Beginning Balance (units)	68,508	76,956	76,956
Net loss	$ (3,726)	$ 118,539
Acquisition of subsidiary interest from noncontrolling interest		(3,123)
Preferred distributions	$ (3,738)	$ (3,738)
DRSPP proceeds (in shares)	5	2
DRSPP proceeds	$ 351	$ 166
Conversion of common units (in shares)		1
Conversion of common units		$ 84
Reallocation of noncontrolling interests in the operating partnership	$ (26,609)	$ 38,529
Deferred compensation plan and stock awards, net of forfeitures and tax withholdings (in shares)	113	(32)
Deferred compensation plan and stock awards, net of forfeitures and tax withholdings	$ 6,728	$ 5,502
Repurchases of common stock (in shares)	(1,306)	(2,562)
Repurchases of common stock	$ (80,310)	$ (219,576)
Issuance of special distribution paid primarily in units	$ 123,529
Issuance of special distribution paid primarily in units (in units)	2,034
Cash distributions declared (per common unit, none of which represented a return of capital for federal income tax purposes)	$ (63,312)	(22,665)
Ending Balance	$ 4,707,991	$ 5,122,402	$ 4,755,078	$ 5,247,868
Ending Balance (units)	69,354	74,365	68,508	76,956
SL Green Operating Partnership | Accumulated Other Comprehensive Loss
Increase (Decrease) in Partner's Capital
Beginning Balance	$ (67,247)	$ (28,485)	$ (28,485)
Other comprehensive income	48,350	(52,383)
Ending Balance	(18,897)	(80,868)	(67,247)	$ (28,485)
SL Green Operating Partnership | Noncontrolling Interests
Increase (Decrease) in Partner's Capital
Beginning Balance	26,032	75,883	75,883
Net loss	(1,499)	(293)
Acquisition of subsidiary interest from noncontrolling interest		1,587
Contribution to consolidated joint venture interests	171	3,814
Cash distributions to noncontrolling interests	(110)	(492)
Ending Balance	24,594	80,499	26,032	75,883
Series I Preferred Stock | SL Green Operating Partnership | Preferred Units
Increase (Decrease) in Partner's Capital
Beginning Balance	221,932	221,932	221,932
Ending Balance	$ 221,932	221,932	221,932	221,932
Cumulative Effect, Period of Adoption, Adjustment | SL Green Operating Partnership
Increase (Decrease) in Partner's Capital
Beginning Balance		(39,184)	(39,184)
Ending Balance				(39,184)
Cumulative Effect, Period of Adoption, Adjustment | SL Green Operating Partnership | Common Stock | Partners' Interest
Increase (Decrease) in Partner's Capital
Beginning Balance		(39,184)	(39,184)
Ending Balance				(39,184)
Cumulative Effect, Period of Adoption, Adjusted Balance | SL Green Operating Partnership
Increase (Decrease) in Partner's Capital
Beginning Balance		5,478,014	5,478,014
Ending Balance				5,478,014
Cumulative Effect, Period of Adoption, Adjusted Balance | SL Green Operating Partnership | Common Stock | Partners' Interest
Increase (Decrease) in Partner's Capital
Beginning Balance		$ 5,208,684	$ 5,208,684
Beginning Balance (units)		76,956	76,956
Ending Balance				$ 5,208,684
Ending Balance (units)				76,956
Cumulative Effect, Period of Adoption, Adjusted Balance | SL Green Operating Partnership | Accumulated Other Comprehensive Loss
Increase (Decrease) in Partner's Capital
Beginning Balance		$ (28,485)	$ (28,485)
Ending Balance				$ (28,485)
Cumulative Effect, Period of Adoption, Adjusted Balance | SL Green Operating Partnership | Noncontrolling Interests
Increase (Decrease) in Partner's Capital
Beginning Balance		75,883	75,883
Ending Balance				75,883
Cumulative Effect, Period of Adoption, Adjusted Balance | Series I Preferred Stock | SL Green Operating Partnership | Preferred Units
Increase (Decrease) in Partner's Capital
Beginning Balance		$ 221,932	$ 221,932
Ending Balance				$ 221,932