Consolidated Statements of Capital - USD ($) shares in Thousands, $ in Thousands	3 Months Ended				6 Months Ended	12 Months Ended
	Jun. 30, 2021	Mar. 31, 2021	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2021	Dec. 31, 2020
Increase (Decrease) in Partner's Capital
Accounting Standards Update				us-gaap:AccountingStandardsUpdate201613Member
Acquisition of subsidiary interest from noncontrolling interest				$ (1,536)
DRSPP proceeds	$ 116	$ 351	$ 198	166
Conversion of common units			8,660	84
Reallocation of noncontrolling interests in the operating partnership	(5,866)	(26,609)	(7,385)	38,529
Deferred compensation plan and stock awards, net of forfeitures and tax withholdings	7,591	6,728	7,991	5,502
Repurchases of common stock	(118,577)	(80,310)	(141,292)	(219,576)
Contribution to consolidated joint venture interests	134	171	4,372	3,814
Cash distributions to noncontrolling interests	$ (63)	$ (110)	$ (104)	$ (492)
Cash distribution declared, per common share (in dollars per share)	$ 0.910	$ 0.910	$ 0.911	$ 0.304
Common Stock
Increase (Decrease) in Partner's Capital
DRSPP proceeds (in shares)	2	5	4	2
Conversion of common units			$ 1
Deferred compensation plan and stock awards, net of forfeitures and tax withholdings (in shares)	(1)	113		(32)
Deferred compensation plan and stock awards, net of forfeitures and tax withholdings		$ 2	$ 1	$ (1)
Repurchases of common stock (in shares)	(1,475)	(1,306)	(2,880)	(2,562)
Repurchases of common stock	$ (15)	$ (13)	$ (30)	$ (26)
Noncontrolling Interests
Increase (Decrease) in Partner's Capital
Acquisition of subsidiary interest from noncontrolling interest				1,587
Contribution to consolidated joint venture interests	134	171	4,372	3,814
Cash distributions to noncontrolling interests	(63)	(110)	(104)	(492)
SL Green Operating Partnership
Increase (Decrease) in Partner's Capital
Beginning Balance	4,935,620	4,935,795	5,343,965	5,517,198	$ 4,935,795	$ 5,517,198
Net income (loss)	109,029	(5,225)	61,204	118,246
Acquisition of subsidiary interest from noncontrolling interest				(1,536)
Other comprehensive income (loss)	(47,966)	48,350	(1,503)	(52,383)
Preferred distributions	(3,737)	(3,738)	(3,737)	(3,738)
DRSPP proceeds	116	351	198	166
Conversion of common units			8,660	84
Reallocation of noncontrolling interests in the operating partnership	(5,866)	(26,609)	(7,385)	38,529
Deferred compensation plan and stock awards, net of forfeitures and tax withholdings	7,591	6,728	7,991	5,502
Repurchases of common stock	(118,577)	(80,310)	(141,292)	(219,576)
Contribution to consolidated joint venture interests	134	171	4,372	3,814
Cash distributions to noncontrolling interests	$ (63)	(110)	$ (104)	$ (492)
Issuance of special distribution paid primarily in units		$ 123,529
Cash distribution declared, per common share (in dollars per share)	$ 0.910	$ 0.910	$ 0.911	$ 0.304
Cash distributions declared (per common unit, none of which represented a return of capital for federal income tax purposes)	$ (62,524)	$ (63,312)	$ (66,607)	$ (22,665)
Ending Balance	4,813,757	4,935,620	5,205,762	5,343,965	4,813,757	4,935,795
SL Green Operating Partnership | Common Stock | Partners' Interest
Increase (Decrease) in Partner's Capital
Beginning Balance	$ 4,707,991	$ 4,755,078	$ 5,122,402	$ 5,247,868	$ 4,755,078	$ 5,247,868
Beginning Balance (units)	69,354	68,508	74,365	76,956	68,508	76,956
Net income (loss)	$ 109,069	$ (3,726)	$ 60,181	$ 118,539
Acquisition of subsidiary interest from noncontrolling interest				(3,123)
Preferred distributions	$ (3,737)	$ (3,738)	$ (3,737)	$ (3,738)
DRSPP proceeds (in shares)	2	5	4	2
DRSPP proceeds	$ 116	$ 351	$ 198	$ 166
Conversion of common units (in shares)			97	1
Conversion of common units			$ 8,660	$ 84
Reallocation of noncontrolling interests in the operating partnership	$ (5,866)	$ (26,609)	$ (7,385)	$ 38,529
Deferred compensation plan and stock awards, net of forfeitures and tax withholdings (in shares)	(1)	113	0	(32)
Deferred compensation plan and stock awards, net of forfeitures and tax withholdings	$ 7,591	$ 6,728	$ 7,991	$ 5,502
Repurchases of common stock (in shares)	(1,475)	(1,306)	(2,880)	(2,562)
Repurchases of common stock	$ (118,577)	$ (80,310)	$ (141,292)	$ (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)	(62,524)	$ (63,312)	(66,607)	(22,665)
Ending Balance	$ 4,634,063	$ 4,707,991	$ 4,980,411	$ 5,122,402	$ 4,634,063	$ 4,755,078
Ending Balance (units)	67,880	69,354	71,586	74,365	67,880	68,508
SL Green Operating Partnership | Accumulated Other Comprehensive Loss
Increase (Decrease) in Partner's Capital
Beginning Balance	$ (18,897)	$ (67,247)	$ (80,868)	$ (28,485)	$ (67,247)	$ (28,485)
Other comprehensive income (loss)	(47,966)	48,350	(1,503)	(52,383)
Ending Balance	(66,863)	(18,897)	(82,371)	(80,868)	(66,863)	(67,247)
SL Green Operating Partnership | Noncontrolling Interests
Increase (Decrease) in Partner's Capital
Beginning Balance	24,594	26,032	80,499	75,883	26,032	75,883
Net income (loss)	(40)	(1,499)	1,023	(293)
Acquisition of subsidiary interest from noncontrolling interest				1,587
Contribution to consolidated joint venture interests	134	171	4,372	3,814
Cash distributions to noncontrolling interests	(63)	(110)	(104)	(492)
Ending Balance	24,625	24,594	85,790	80,499	24,625	26,032
Series I Preferred Stock | SL Green Operating Partnership | Preferred Units
Increase (Decrease) in Partner's Capital
Beginning Balance	221,932	221,932	221,932	221,932	221,932	221,932
Ending Balance	$ 221,932	$ 221,932	$ 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)
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)
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
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
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)
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
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