Segment Information - Schedule of Reconciliation of Income from Continuing Operations to Net Income Attributable to SL Green Common Stockholders (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Reconciliation of income from continuing operations to net income attributable to SL Green common stockholders
Net income (loss) before equity in net gain on sale of interest in unconsolidated joint venture/real estate, purchase price and other fair value adjustments, (loss) gain on sale of real estate net, depreciable real estate reserves, and gain on sale of marketable securities	$ 47,515	$ 35,813	$ 94,964	$ 87,133
Equity in net gain on sale of interest in unconsolidated joint venture/real estate	72,025	13,089	65,585	15,136
Purchase price and other fair value adjustments	11,149	0	60,442	0
(Loss) gain on sale of real estate, net	(14,790)	(3,823)	8,731	(3,256)
Depreciable real estate reserves	0	(29,064)	0	(85,336)
Gain on sale of investment in marketable securities	0	0	0	3,262
Net income	$ 115,899	$ 16,015	$ 229,722	$ 16,939