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Consolidated Statement of Capital - USD ($) shares in Thousands, $ in Thousands	3 Months Ended				12 Months Ended
	Dec. 31, 2019	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Jan. 01, 2018
Increase (Decrease) in Partner's Capital
Preferred dividends	$ (3,086)	$ (5,239)	$ 838	$ (8,319)
DRSPP proceeds					$ 334	$ 136	$ 223
Reallocation of noncontrolling interests in the operating partnership					(34,320)	34,236	5,712
Equity component of repurchased exchangeable senior notes							(109,776)
Deferred compensation plan and stock awards, net of forfeitures and tax withholdings					25,763	17,484	29,787
Repurchases of common units					(384,399)	(937,795)	(848,048)
Deconsolidation of partially owned entity						(315,116)	(30,203)
Cash distributions to noncontrolling interests					(478)	(8,364)	(52,446)
Conversion of common units					471	16,303	21,574
Redemption of common units					$ (25,845)
Proceeds from stock options exercised						$ 28,912	$ 23,314
Common Stock
Increase (Decrease) in Partner's Capital
DRSPP proceeds (in units)					4	1	2
DRSPP proceeds
Deferred compensation plan and stock awards, net of forfeitures and tax withholdings (in units)					105	149	87
Deferred compensation plan and stock awards, net of forfeitures and tax withholdings					$ 2	$ 1	$ 1
Repurchases of common units (in units)					(4,596)	(9,745)	(8,342)
Repurchases of common units					$ (46)	$ (98)	$ (83)
Conversion of common units						$ 2	$ 2
Proceeds from stock options exercised (in shares)						316	292
Proceeds from stock options exercised						$ 3	$ 2
Noncontrolling Interests
Increase (Decrease) in Partner's Capital
Deconsolidation of partially owned entity						(315,116)	(30,203)
Cash distributions to noncontrolling interests					(478)	(8,364)	(52,446)
Redemption of common units					(25,276)
SL Green Operating Partnership
Increase (Decrease) in Partner's Capital
Beginning Balance		5,947,855		6,589,454	5,947,855	6,589,454	7,750,911
Adjusted Balance							7,159,978
Net income					267,275	247,256	85,673
Other comprehensive income (loss)					(43,593)	(3,496)	(3,533)
Preferred dividends	(2,091)	(2,961)	(2,601)	(3,047)	(14,950)	(14,950)	(14,950)
DRSPP proceeds					334	136	223
Reallocation of noncontrolling interests in the operating partnership					(34,320)	34,236	5,712
Equity component of repurchased exchangeable senior notes							(109,776)
Deferred compensation plan and stock awards, net of forfeitures and tax withholdings					25,763	17,484	29,787
Repurchases of common units					(384,399)	(937,795)
Proceeds from stock options exercised							23,314
Contributions to consolidated joint venture interests					58,462	5,459	36,275
Deconsolidation of partially owned entity						(315,116)
Cash distributions to noncontrolling interests					(478)	(8,364)	(52,446)
Cash distributions declared (none of which represented a return of capital for federal income tax purposes)					(279,377)	(282,188)	(305,059)
Conversion of common units					471	16,303	21,574
Redemption of common units					(25,845)
Proceeds from stock options exercised						28,912
Ending Balance	5,517,198		5,947,855		5,517,198	5,947,855	6,589,454
SL Green Operating Partnership \| Preferred Units \| Series I Preferred Stock
Increase (Decrease) in Partner's Capital
Beginning Balance		221,932		221,932	221,932	221,932	221,932
Adjusted Balance							221,932
Ending Balance	221,932		221,932		221,932	221,932	221,932
SL Green Operating Partnership \| Common Stock \| Partners' Interest
Increase (Decrease) in Partner's Capital
Beginning Balance		$ 5,664,481		$ 5,984,557	$ 5,664,481	$ 5,984,557	$ 7,080,406
Beginning Balance (units)		83,684		92,803	83,684	92,803	100,562
Adjusted Balance							$ 6,555,081
Net income					$ 270,434	$ 247,262	101,374
Preferred dividends					$ (14,950)	$ (14,950)	$ (14,950)
DRSPP proceeds (in units)					4	1	2
DRSPP proceeds					$ 334	$ 136	$ 223
Reallocation of noncontrolling interests in the operating partnership					$ (34,320)	$ 34,236	5,712
Equity component of repurchased exchangeable senior notes							$ (109,776)
Deferred compensation plan and stock awards, net of forfeitures and tax withholdings (in units)					105	149	87
Deferred compensation plan and stock awards, net of forfeitures and tax withholdings					$ 25,763	$ 17,484	$ 29,787
Repurchases of common units (in units)					(4,596)	(9,745)	(8,342)
Repurchases of common units					$ (384,399)	$ (937,795)	$ (848,048)
Proceeds from stock options exercised (in units)							292
Proceeds from stock options exercised							$ 23,314
Cash distributions declared (none of which represented a return of capital for federal income tax purposes)					$ (279,377)	$ (282,188)	$ (305,059)
Conversion of common units (in units)					5	160	202
Conversion of common units					$ 471	$ 16,303	$ 21,574
Redemption of common units					(569)
Proceeds from stock options exercised (in shares)						316
Proceeds from stock options exercised						$ 28,912
Ending Balance	$ 5,247,868		$ 5,664,481		$ 5,247,868	$ 5,664,481	$ 5,984,557
Ending Balance (units)	79,202		83,684		79,202	83,684	92,803
SL Green Operating Partnership \| Accumulated Other Comprehensive Income (Loss)
Increase (Decrease) in Partner's Capital
Beginning Balance		$ 15,108		$ 18,604	$ 15,108	$ 18,604	$ 22,137
Adjusted Balance							18,604
Other comprehensive income (loss)					(43,593)	(3,496)	(3,533)
Ending Balance	$ (28,485)		$ 15,108		(28,485)	15,108	18,604
SL Green Operating Partnership \| Noncontrolling Interests
Increase (Decrease) in Partner's Capital
Beginning Balance		$ 46,334		$ 364,361	46,334	364,361	426,436
Adjusted Balance							364,361
Net income					(3,159)	(6)	(15,701)
Contributions to consolidated joint venture interests					58,462	5,459	36,275
Deconsolidation of partially owned entity						(315,116)	(30,203)
Cash distributions to noncontrolling interests					(478)	(8,364)	(52,446)
Redemption of common units					(25,276)
Ending Balance	$ 75,883		$ 46,334		$ 75,883	$ 46,334	$ 364,361
Accounting Standards Update 2014-09
Increase (Decrease) in Partner's Capital
Cumulative adjustment upon adoption of ASC 610-20								$ 570,524
Accounting Standards Update 2014-09 \| SL Green Operating Partnership
Increase (Decrease) in Partner's Capital
Cumulative adjustment upon adoption of ASC 610-20								570,524
Accounting Standards Update 2014-09 \| SL Green Operating Partnership \| Common Stock \| Partners' Interest
Increase (Decrease) in Partner's Capital
Cumulative adjustment upon adoption of ASC 610-20								$ 570,524