Accumulated Other Comprehensive Loss of the Company (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2015	Dec. 31, 2014
Accumulated Other Comprehensive Income (Loss) of the Company [Roll Forward]
Beginning Balance	$ (6,980)
Ending Balance	(15,821)
Deferred net losses from terminated hedges	10,300	$ 11,800
SL Green Realty Corp
Accumulated Other Comprehensive Income (Loss) of the Company [Roll Forward]
Beginning Balance	(6,980)
Other comprehensive loss before reclassifications	(17,662)
Amounts reclassified from accumulated other comprehensive income	8,821
Ending Balance	(15,821)
SL Green Realty Corp | Net unrealized (loss) gain on derivative instruments
Accumulated Other Comprehensive Income (Loss) of the Company [Roll Forward]
Beginning Balance	(9,498)
Other comprehensive loss before reclassifications	(14,918)
Amounts reclassified from accumulated other comprehensive income	7,908
Ending Balance	(16,508)
SL Green Realty Corp | Unrealized gain (loss) on marketable securities
Accumulated Other Comprehensive Income (Loss) of the Company [Roll Forward]
Beginning Balance	2,613
Other comprehensive loss before reclassifications	(351)
Amounts reclassified from accumulated other comprehensive income	0
Ending Balance	2,262
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	(95)
Other comprehensive loss before reclassifications	(2,393)
Amounts reclassified from accumulated other comprehensive income	913
Ending Balance	$ (1,575)