Consolidated Statement of Capital (Unaudited) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Increase (Decrease) in Partner's Capital
DRSPP proceeds	$ 52	$ 74	$ 64	$ 111
Conversion of common units	0	2,111	1,560	13,242
Reallocation of noncontrolling interests in the operating partnership	11,941	15,381	(4,493)	(12,712)
Equity component of repurchased exchangeable senior notes		(9,145)		(18,824)
Deferred compensation plan and stock awards, net of forfeitures and tax withholdings	4,634	6,511	8,090	13,544
Repurchases of common stock	(25,236)	(96,794)	(693,091)	(253,197)
Contribution to consolidated joint venture interests	3,072	193	1,828	33,009
Deconsolidation of partially owned entity	(520)	0	(314,596)	(30,203)
Cash distributions to noncontrolling interests	$ (1,139)	$ (44,480)	$ (724)	$ (609)
Common Stock
Increase (Decrease) in Partner's Capital
DRSPP proceeds (in shares)	0	1	1	1
Conversion of common units		$ 0		$ 1
Deferred compensation plan and stock awards, net of forfeitures and tax withholdings (in shares)	13	2	(20)	(8)
Repurchases of common stock (in shares)	(253)	(952)	(7,133)	(2,448)
Repurchases of common stock	$ (2)	$ (10)	$ (72)	$ (24)
Contributions - proceeds from stock options exercised (in shares)	109	8	59	138
Noncontrolling Interests
Increase (Decrease) in Partner's Capital
Contribution to consolidated joint venture interests	$ 3,072	$ 193	$ 1,828	$ 33,009
Deconsolidation of partially owned entity	(520)	0	(314,596)	(30,203)
Cash distributions to noncontrolling interests	(1,139)	(44,480)	(724)	(609)
SL Green Operating Partnership
Increase (Decrease) in Partner's Capital
Beginning Balance	6,242,056	7,347,640	6,589,454	7,750,911
Adjusted Balance			7,159,978
Net income	91,811	41,133	213,168	10,343
Other comprehensive income	3,677	(169)	14,018	(7,783)
Preferred distributions	(3,738)	(3,738)	(7,475)	(7,475)
DRSPP proceeds	52	74	64	111
Conversion of common units	0	2,111	1,560	13,242
Reallocation of noncontrolling interests in the operating partnership	11,941	15,381	(4,493)	(12,712)
Equity component of repurchased exchangeable senior notes		(9,145)		(18,824)
Deferred compensation plan and stock awards, net of forfeitures and tax withholdings	4,634	6,511	8,090	13,544
Repurchases of common stock	(25,236)	(96,794)	(693,091)	(253,197)
Contribution to consolidated joint venture interests	3,072	193	1,828	33,009
Deconsolidation of partially owned entity	(520)	0	(314,596)	(30,203)
Contributions - proceeds from stock options exercised	9,936	695	5,637	11,595
Cash distributions to noncontrolling interests	(1,139)	(44,480)	(724)	(609)
Cash distributions declared (per common unit, none of which represented a return of capital for federal income tax purposes)	(69,371)	(75,360)	(141,908)	(154,312)
Ending Balance	6,267,175	7,184,052	6,242,056	7,347,640
SL Green Operating Partnership | Common Stock | Partners' Interest
Increase (Decrease) in Partner's Capital
Beginning Balance	$ 5,936,262	$ 6,699,426	$ 5,984,557	$ 7,080,406
Beginning Balance (units)	85,725	98,367	92,803	100,562
Adjusted Balance			$ 6,555,081
Net income	$ 91,947	$ 42,607	212,797	$ 27,048
Preferred distributions	$ (3,738)	$ (3,738)	$ (7,475)	$ (7,475)
DRSPP proceeds (in shares)	0	1	1	1
DRSPP proceeds	$ 52	$ 74	$ 64	$ 111
Conversion of common units (in shares)		20	15	122
Conversion of common units		$ 2,111	$ 1,560	$ 13,242
Reallocation of noncontrolling interests in the operating partnership	$ 11,941	15,381	$ (4,493)	(12,712)
Equity component of repurchased exchangeable senior notes		$ (9,145)		$ (18,824)
Deferred compensation plan and stock awards, net of forfeitures and tax withholdings (in shares)	13	2	(20)	(8)
Deferred compensation plan and stock awards, net of forfeitures and tax withholdings	$ 4,634	$ 6,511	$ 8,090	$ 13,544
Repurchases of common stock (in shares)	(253)	(952)	(7,133)	(2,448)
Repurchases of common stock	$ (25,236)	$ (96,794)	$ (693,091)	$ (253,197)
Contributions - proceeds from stock options exercised (in shares)	109	8	59	138
Contributions - proceeds from stock options exercised	$ 9,936	$ 695	$ 5,637	$ 11,595
Cash distributions declared (per common unit, none of which represented a return of capital for federal income tax purposes)	(69,371)	(75,360)	(141,908)	(154,312)
Ending Balance	$ 5,956,427	$ 6,581,768	$ 5,936,262	$ 6,699,426
Ending Balance (units)	85,594	97,446	85,725	98,367
SL Green Operating Partnership | Accumulated Other Comprehensive Income
Increase (Decrease) in Partner's Capital
Beginning Balance	$ 32,622	$ 14,354	$ 18,604	$ 22,137
Adjusted Balance			18,604
Other comprehensive income	3,677	(169)	14,018	(7,783)
Ending Balance	36,299	14,185	32,622	14,354
SL Green Operating Partnership | Noncontrolling Interests
Increase (Decrease) in Partner's Capital
Beginning Balance	51,240	411,928	364,361	426,436
Adjusted Balance			364,361
Net income	(136)	(1,474)	371	(16,705)
Contribution to consolidated joint venture interests	3,072	193	1,828	33,009
Deconsolidation of partially owned entity	(520)		(314,596)	(30,203)
Cash distributions to noncontrolling interests	(1,139)	(44,480)	(724)	(609)
Ending Balance	52,517	366,167	51,240	411,928
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
Adjusted Balance			221,932
Ending Balance	$ 221,932	$ 221,932	$ 221,932	$ 221,932
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