Consolidated Statements of Capital - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended		9 Months Ended	12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Jun. 30, 2021	Jun. 30, 2020	Sep. 30, 2021	Dec. 31, 2020	Dec. 31, 2019
Increase (Decrease) in Partner's Capital
Accounting Standards Update							us-gaap:AccountingStandardsUpdate201613Member
Acquisition of subsidiary interest from noncontrolling interest				$ (1,536)
DRSPP proceeds	$ 146	$ 202	$ 467	364
Conversion of common units		0		8,744
Reallocation of noncontrolling interests in the operating partnership	15,230	4,109	(32,475)	31,144
Deferred compensation plan and stock awards, net of forfeitures and tax withholdings	7,932	7,543	14,319	13,493
Repurchases of common stock	(77,957)	(31,127)	(198,887)	(360,868)
Contribution to consolidated joint venture interests	5	4,219	305	8,186
Sale of interest in partially owned entity	(4,476)
Cash distributions to noncontrolling interests	$ (5,230)	$ (12,530)	$ (173)	$ (596)
Cash distribution (in dollars per share)	$ 0.910	$ 0.911	$ 1.820	$ 1.214
Common Stock
Increase (Decrease) in Partner's Capital
DRSPP proceeds (in shares)	2	4	7	6
Conversion of common units				$ 1
Deferred compensation plan and stock awards, net of forfeitures and tax withholdings (in shares)	(1)	3	112	(32)
Deferred compensation plan and stock awards, net of forfeitures and tax withholdings			$ 2
Repurchases of common stock (in shares)	(1,016)	(624)	(2,781)	(5,442)
Repurchases of common stock	$ (10)	$ (7)	$ (28)	$ (56)
Noncontrolling Interests
Increase (Decrease) in Partner's Capital
Acquisition of subsidiary interest from noncontrolling interest				1,587
Contribution to consolidated joint venture interests	5	4,219	305	8,186
Sale of interest in partially owned entity	(4,476)
Cash distributions to noncontrolling interests	(5,230)	(12,530)	(173)	(596)
SL Green Operating Partnership
Increase (Decrease) in Partner's Capital
Beginning Balance	4,813,757	5,205,762	4,935,795	5,517,198	$ 4,935,795	$ 5,517,198
Net income (loss)	391,028	18,011	103,804	179,450
Acquisition of subsidiary interest from noncontrolling interest				(1,536)
Other comprehensive income (loss)	6,266	6,171	384	(53,886)
Preferred distributions	(3,738)	(3,738)	(7,475)	(7,475)
DRSPP proceeds	146	202	467	364
Conversion of common units		0		8,744
Reallocation of noncontrolling interests in the operating partnership	15,230	4,109	(32,475)	31,144
Deferred compensation plan and stock awards, net of forfeitures and tax withholdings	7,932	7,543	14,319	13,493
Repurchases of common stock	(77,957)	(31,127)	(198,887)	(360,868)
Contribution to consolidated joint venture interests	5	4,219	305	8,186
Sale of interest in partially owned entity	(4,476)
Cash distributions to noncontrolling interests	(5,230)	$ (12,530)	(173)	$ (596)
Issuance of special distribution paid primarily in units	$ 0		$ 123,529
Cash distribution (in dollars per share)	$ 0.910	$ 0.911	$ 1.820	$ 1.214
Cash distributions declared (per common unit, none of which represented a return of capital for federal income tax purposes)	$ (60,748)	$ (64,617)	$ (125,836)	$ (89,272)
Ending Balance	5,082,215	5,134,005	4,813,757	5,205,762	5,082,215	4,935,795	$ 5,517,198
SL Green Operating Partnership | Common Stock | Partners' Interest
Increase (Decrease) in Partner's Capital
Beginning Balance	$ 4,634,063	$ 4,980,411	$ 4,755,078	$ 5,247,868	$ 4,755,078	$ 5,247,868
Beginning Balance (units)	67,880	71,586	68,508	76,956	68,508	76,956
Net income (loss)	$ 391,943	$ 17,597	$ 105,343	$ 178,720
Acquisition of subsidiary interest from noncontrolling interest				(3,123)
Preferred distributions	$ (3,738)	$ (3,738)	$ (7,475)	$ (7,475)
DRSPP proceeds (in shares)	2	4	7	6
DRSPP proceeds	$ 146	$ 202	$ 467	$ 364
Conversion of common units (in shares)		0		98
Conversion of common units		$ 0		$ 8,744
Reallocation of noncontrolling interests in the operating partnership	$ 15,230	$ 4,109	$ (32,475)	$ 31,144
Deferred compensation plan and stock awards, net of forfeitures and tax withholdings (in shares)	(1)	3	112	(32)
Deferred compensation plan and stock awards, net of forfeitures and tax withholdings	$ 7,932	$ 7,543	$ 14,319	$ 13,493
Repurchases of common stock (in shares)	(1,016)	(624)	(2,781)	(5,442)
Repurchases of common stock	$ (77,957)	$ (31,127)	$ (198,887)	$ (360,868)
Issuance of special distribution paid primarily in units	0		$ 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)	(60,748)	(64,617)	$ (125,836)	(89,272)
Ending Balance	$ 4,906,871	$ 4,910,380	$ 4,634,063	$ 4,980,411	$ 4,906,871	$ 4,755,078	$ 5,247,868
Ending Balance (units)	66,865	70,969	67,880	71,586	66,865	68,508	76,956
SL Green Operating Partnership | Accumulated Other Comprehensive Loss
Increase (Decrease) in Partner's Capital
Beginning Balance	$ (66,863)	$ (82,371)	$ (67,247)	$ (28,485)	$ (67,247)	$ (28,485)
Other comprehensive income (loss)	6,266	6,171	384	(53,886)
Ending Balance	(60,597)	(76,200)	(66,863)	(82,371)	(60,597)	(67,247)	$ (28,485)
SL Green Operating Partnership | Noncontrolling Interests
Increase (Decrease) in Partner's Capital
Beginning Balance	24,625	85,790	26,032	75,883	26,032	75,883
Net income (loss)	(915)	414	(1,539)	730
Acquisition of subsidiary interest from noncontrolling interest				1,587
Contribution to consolidated joint venture interests	5	4,219	305	8,186
Sale of interest in partially owned entity	(4,476)
Cash distributions to noncontrolling interests	(5,230)	(12,530)	(173)	(596)
Ending Balance	14,009	77,893	24,625	85,790	14,009	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	221,932	221,932	221,932
Ending Balance	$ 221,932	$ 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)
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