Consolidated Statement of Capital - USD ($) shares in Thousands, $ in Thousands	3 Months Ended				12 Months Ended
	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Increase (Decrease) in Partner's Capital
Acquisition of subsidiary interest from noncontrolling interest							$ (20,650)
Preferred distributions	$ (6,616)	$ 14,165	$ (6,217)	$ (5,544)
Conversion of common units					$ 21,574	$ 31,806	55,697
DRSPP proceeds					223	277	99,555
Reallocation of capital account relating to sale							(10,143)
Reallocation of noncontrolling interests in the operating partnership					5,712	(4,222)	20,915
Equity component of repurchased exchangeable senior notes					(109,776)
Amortization of deferred compensation plan					32,161	25,890	26,721
Proceeds from stock options exercised					23,314	14,832	16,362
Deconsolidation of partially owned entity					(30,203)
Cash distributions to noncontrolling interests					(52,446)	(15,419)	(119,784)
Common Stock
Increase (Decrease) in Partner's Capital
Conversion of common units					$ 2	$ 3	$ 5
DRSPP (in units)					2	2	776
DRSPP proceeds							$ 8
Proceeds from stock options exercised (in shares)					292	193	217
Proceeds from stock options exercised					$ 2	$ 2	$ 2
Contributions - net proceeds from common stock offering (in units)							1,007
Noncontrolling Interests
Increase (Decrease) in Partner's Capital
Acquisition of subsidiary interest from noncontrolling interest							$ (11,084)
Reallocation of capital account relating to sale							(10,143)
Deconsolidation of partially owned entity					(30,203)
Cash distributions to noncontrolling interests					(52,446)	$ (15,419)	(119,784)
SL Green Operating Partnership
Increase (Decrease) in Partner's Capital
Beginning Balance		7,750,911		7,719,317	7,750,911	7,719,317	7,459,216
Net income					85,673	257,540	299,927
Acquisition of subsidiary interest from noncontrolling interest							(20,650)
Other comprehensive (loss)					(3,533)	30,886	(1,769)
Preferred distributions	(5,328)	14,641	(4,252)	(4,622)	(14,950)	(14,950)	(14,952)
Conversion of common units					21,574	31,806	55,697
DRSPP proceeds					223	277	99,555
Reallocation of capital account relating to sale							(10,143)
Reallocation of noncontrolling interests in the operating partnership					5,712	(4,222)	20,915
Equity component of repurchased exchangeable senior notes					(109,776)
Deferred compensation plan and stock award, net					(2,374)	(1,988)	(2,982)
Amortization of deferred compensation plan					32,161	25,890	26,721
Repurchases of common stock					(848,048)
Proceeds from stock options exercised					23,314
Issuance of stock						(40)
Contribution to consolidated joint venture interests					36,275	2,359	35,178
Deconsolidation of partially owned entity					(30,203)
Contributions - net proceeds from common stock offering							126,989
Contributions - proceeds from stock options exercised						14,832	16,362
Cash distributions to noncontrolling interests					(52,446)	(15,419)	(119,784)
Cash distributions declared ($3.1375, $2.94, and $2.52 per common unit, respectively, none of which represented a return of capital for federal income tax purposes)					(305,059)	(295,377)	(250,963)
Ending Balance	6,589,454		7,750,911		6,589,454	7,750,911	7,719,317
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
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		$ 7,080,406		$ 7,074,282	$ 7,080,406	$ 7,074,282	$ 6,722,422
Beginning Balance (units)		100,562		99,976	100,562	99,976	97,325
Net income					$ 101,374	$ 249,896	$ 284,084
Acquisition of subsidiary interest from noncontrolling interest							(9,566)
Preferred distributions					$ (14,950)	$ (14,950)	$ (14,952)
Conversion of common units (in units)					202	295	483
Conversion of common units					$ 21,574	$ 31,806	$ 55,697
DRSPP (in units)					2		776
DRSPP proceeds					$ 223	277	$ 99,555
Reallocation of noncontrolling interests in the operating partnership					5,712	$ (4,222)	$ 20,915
Equity component of repurchased exchangeable senior notes					$ (109,776)
Deferred compensation plan and stock award, net, (in units)					87	96	168
Deferred compensation plan and stock award, net					$ (2,374)	$ (1,988)	$ (2,982)
Amortization of deferred compensation plan					$ 32,161	25,890	$ 26,721
Repurchases of common stock (in units)					(8,342)
Proceeds from stock options exercised (in shares)					292
Proceeds from stock options exercised					$ 23,314
Issuance of stock						$ 40
Contributions - net proceeds from common stock offering (in units)							1,007
Contributions - net proceeds from common stock offering							$ 126,989
Contributions - proceeds from stock options exercised (in units)						193	217
Contributions - proceeds from stock options exercised						$ 14,832	$ 16,362
Cash distributions declared ($3.1375, $2.94, and $2.52 per common unit, respectively, none of which represented a return of capital for federal income tax purposes)					(305,059)	295,377	(250,963)
Ending Balance	$ 5,984,557		$ 7,080,406		$ 5,984,557	$ 7,080,406	$ 7,074,282
Ending Balance (units)	92,803		100,562		92,803	100,562	99,976
SL Green Operating Partnership | Accumulated Other Comprehensive Income (Loss)
Increase (Decrease) in Partner's Capital
Beginning Balance		$ 22,137		$ (8,749)	$ 22,137	$ (8,749)	$ (6,980)
Other comprehensive (loss)					(3,533)	30,886	(1,769)
Ending Balance	$ 18,604		$ 22,137		18,604	22,137	(8,749)
SL Green Operating Partnership | Noncontrolling Interests
Increase (Decrease) in Partner's Capital
Beginning Balance		$ 426,436		$ 431,852	426,436	431,852	521,842
Net income					(15,701)	7,644	15,843
Acquisition of subsidiary interest from noncontrolling interest							(11,084)
Reallocation of capital account relating to sale							(10,143)
Contribution to consolidated joint venture interests					36,275	2,359	35,178
Deconsolidation of partially owned entity					(30,203)
Cash distributions to noncontrolling interests					(52,446)	15,419	(119,784)
Ending Balance	$ 364,361		$ 426,436		$ 364,361	$ 426,436	$ 431,852