Accumulated Other Comprehensive Income (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2017	Dec. 31, 2016
Accumulated Other Comprehensive Income (Loss) of the Company [Roll Forward]
Beginning Balance	$ 7,750,911
Ending Balance	7,184,052
Accumulated Other Comprehensive Income (Loss)
Accumulated Other Comprehensive Income (Loss) of the Company [Roll Forward]
Beginning Balance	22,137
Ending Balance	14,185
SL Green Realty Corp
Accumulated Other Comprehensive Income (Loss) of the Company [Roll Forward]
Other comprehensive (loss) before reclassifications	(6,749)
Amounts reclassified from accumulated other comprehensive income	(1,203)
Deferred net losses from terminated hedges	3,700	$ 7,100
SL Green Realty Corp | Net unrealized (loss) gain on derivative instruments
Accumulated Other Comprehensive Income (Loss) of the Company [Roll Forward]
Beginning Balance	12,596
Other comprehensive (loss) before reclassifications	(4,669)
Amounts reclassified from accumulated other comprehensive income	1,336
Ending Balance	9,263
SL Green Realty Corp | Net unrealized gain on marketable securities
Accumulated Other Comprehensive Income (Loss) of the Company [Roll Forward]
Beginning Balance	5,520
Other comprehensive (loss) before reclassifications	(1,133)
Amounts reclassified from accumulated other comprehensive income	(3,130)
Ending Balance	1,257
SL Green Realty Corp | Accumulated Other Comprehensive Income (Loss)
Accumulated Other Comprehensive Income (Loss) of the Company [Roll Forward]
Beginning Balance	22,137
Ending Balance	14,185
SL Green Realty Corp | Joint venture | Net unrealized (loss) gain on derivative instruments
Accumulated Other Comprehensive Income (Loss) of the Company [Roll Forward]
Beginning Balance	4,021
Other comprehensive (loss) before reclassifications	(947)
Amounts reclassified from accumulated other comprehensive income	591
Ending Balance	$ 3,665