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		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Reconciliation of income from continuing operations to net income attributable to SL Green common stockholders
Net income 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, loss on early extinguishment of debt, and gain on sale of marketable securities	$ 99,454	$ 45,795	$ 329,176	$ 62,734
Equity in net gain on sale of interest in unconsolidated joint venture/real estate	70,937	1,030	136,522	16,166
Purchase price and other fair value adjustments	(3,057)	0	57,385	0
(Loss) gain on sale of real estate, net	(2,504)	0	6,227	(3,256)
Depreciable real estate reserves	(6,691)	0	(6,691)	(85,336)
Loss on early extinguishment of debt	(2,194)	0	(2,194)	0
Gain on sale of investment in marketable securities	0	0	0	3,262
Net income	$ 99,454	$ 45,795	$ 329,176	$ 62,734