Consolidated Statement of Capital - USD ($) shares in Thousands, $ in Thousands	3 Months Ended				12 Months Ended
	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Jan. 01, 2018
Increase (Decrease) in Partner's Capital
Preferred dividends	$ 838	$ (8,319)	$ (6,616)	$ 14,165
Conversion of common units					$ 16,303	$ 21,574	$ 31,806
DRSPP proceeds					136	223	277
Reallocation of noncontrolling interests in the operating partnership					34,236	5,712	(4,222)
Equity component of repurchased exchangeable senior notes						(109,776)
Proceeds from stock options exercised					28,912	23,314	14,832
Deconsolidation of partially owned entity					(315,116)	(30,203)
Cash distributions to noncontrolling interests					(8,364)	(52,446)	(15,419)
Common Stock
Increase (Decrease) in Partner's Capital
Conversion of common units					$ 2	$ 2	$ 3
DRSPP (in units)					1	2	2
DRSPP proceeds
Proceeds from stock options exercised (in shares)					316	292	193
Proceeds from stock options exercised					$ 3	$ 2	$ 2
Noncontrolling Interests
Increase (Decrease) in Partner's Capital
Contributions to consolidated joint venture interests						36,275
Deconsolidation of partially owned entity					(315,116)	(30,203)
Cash distributions to noncontrolling interests					(8,364)	(52,446)	(15,419)
SL Green Operating Partnership
Increase (Decrease) in Partner's Capital
Beginning Balance		6,589,454		7,750,911	6,589,454	7,750,911	7,719,317
Adjusted Balance			7,159,978			7,159,978
Net income					247,256	85,673	257,540
Other comprehensive income (loss)					(3,496)	(3,533)	30,886
Preferred dividends	(2,601)	(3,047)	(5,328)	14,641	(14,950)	(14,950)	(14,950)
Conversion of common units					16,303	21,574	31,806
DRSPP proceeds					136	223	277
Reallocation of noncontrolling interests in the operating partnership					34,236	5,712	(4,222)
Equity component of repurchased exchangeable senior notes						(109,776)
Deferred compensation plan and stock awards, net of forfeitures and tax withholdings					17,484	29,787	23,902
Repurchases of common units					(937,795)	(848,048)
Proceeds from stock options exercised					28,912	23,314
Issuance of stock							(40)
Contributions to consolidated joint venture interests					5,459	36,275	2,359
Deconsolidation of partially owned entity					(315,116)	(30,203)
Proceeds from stock options exercised							14,832
Cash distributions to noncontrolling interests					(8,364)	(52,446)	(15,419)
Cash distributions declared (none of which represented a return of capital for federal income tax purposes)					(282,188)	(305,059)	(295,377)
Ending Balance	5,947,855		6,589,454		5,947,855	6,589,454	7,750,911
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			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		$ 5,984,557		$ 7,080,406	$ 5,984,557	$ 7,080,406	$ 7,074,282
Beginning Balance (units)		92,803		100,562	92,803	100,562	99,976
Adjusted Balance			6,555,081			$ 6,555,081
Net income					$ 247,262	101,374	$ 249,896
Preferred dividends					$ (14,950)	$ (14,950)	$ (14,950)
Conversion of common units (in units)					160	202	295
Conversion of common units					$ 16,303	$ 21,574	$ 31,806
DRSPP (in units)					1		2
DRSPP proceeds					$ 136	223	$ 277
Reallocation of noncontrolling interests in the operating partnership					$ 34,236	5,712	$ (4,222)
Equity component of repurchased exchangeable senior notes						$ (109,776)
Deferred compensation plan and stock award, net, (in units)					149	87	96
Deferred compensation plan and stock awards, net of forfeitures and tax withholdings					$ 17,484	$ 29,787	$ 23,902
Repurchases of common stock (in units)					(9,745)	(8,342)
Repurchases of common units					$ (937,795)	$ (848,048)
Proceeds from stock options exercised (in shares)					316	292
Proceeds from stock options exercised					$ 28,912	$ 23,314
Issuance of stock							$ (40)
Proceeds from stock options exercised (in units)							193
Proceeds from stock options exercised							$ 14,832
Cash distributions declared (none of which represented a return of capital for federal income tax purposes)					(282,188)	(305,059)	(295,377)
Ending Balance	$ 5,664,481		$ 5,984,557		$ 5,664,481	$ 5,984,557	$ 7,080,406
Ending Balance (units)	83,684		92,803		83,684	92,803	100,562
SL Green Operating Partnership | Accumulated Other Comprehensive Income (Loss)
Increase (Decrease) in Partner's Capital
Beginning Balance		$ 18,604		$ 22,137	$ 18,604	$ 22,137	$ (8,749)
Adjusted Balance			$ 18,604			18,604
Other comprehensive income (loss)					(3,496)	(3,533)	30,886
Ending Balance	$ 15,108		18,604		15,108	18,604	22,137
SL Green Operating Partnership | Noncontrolling Interests
Increase (Decrease) in Partner's Capital
Beginning Balance		$ 364,361		$ 426,436	364,361	426,436	431,852
Adjusted Balance			364,361			364,361
Net income					(6)	(15,701)	7,644
Contributions to consolidated joint venture interests					5,459		2,359
Deconsolidation of partially owned entity					(315,116)
Cash distributions to noncontrolling interests					(8,364)	(52,446)	(15,419)
Ending Balance	$ 46,334		$ 364,361		$ 46,334	$ 364,361	$ 426,436
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