Stockholders' Equity of the Company - Earnings per Share (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Basic Earnings:
Income attributable to SL Green common stockholders	$ 27,982	$ 38,869	$ 8,222	$ 11,351	$ 44,016	$ 34,252	$ 133,457	$ 23,221	$ 86,424	$ 234,946	$ 269,132
Effect of Dilutive Securities:
Redemption of units to common shares									3,995	10,136	10,565
Diluted Earnings:
Income attributable to SL Green common stockholders									$ 90,419	$ 245,082	$ 279,697
Basic Shares:
Weighted average common shares outstanding (shares)									98,571,000	100,185,000	99,345,000
Effect of Dilutive Securities:
Redemption of units to common shares (shares)									4,556,000	4,323,000	3,899,000
Stock-based compensation plans (shares)									276,000	373,000	490,000
Diluted weighted average common stock outstanding (shares)									103,403,000	104,881,000	103,734,000
Common stock shares excluded from the diluted shares outstanding (shares)									1,092,870	774,782	263,991