Noncontrolling Interests on the Company's Consolidated Financial Statements - Additional Information (Details)	1 Months Ended	12 Months Ended
	Jan. 31, 2012 $ / shares shares	Dec. 31, 2018 USD ($) shares $ / shares	Dec. 31, 2017 USD ($) unit $ / shares shares	Dec. 31, 2016 USD ($) shares	Mar. 31, 2016 shares	Aug. 31, 2015 shares	Jul. 31, 2015 shares	Jun. 30, 2015 shares	Feb. 28, 2015 shares	Aug. 31, 2014 shares
Organization
Basic weighted average common shares outstanding (in shares)		86,753,000	98,571,000	100,185,000
Rollforward analysis of the activity relating to the noncontrolling interests in the operating partnership
Balance at beginning of period | $		$ 461,954,000
Distributions | $		(8,364,000)	$ (52,446,000)	$ (15,419,000)
Net income | $		12,216,000	3,995,000	10,136,000
Accumulated other comprehensive income allocation | $		(66,000)	(94,000)	$ 1,299,000
Balance at end of period | $		$ 387,805,000	$ 461,954,000
Number of company common stock issued on conversion of Series B preferred units		6.71348
Diluted weighted average common stock outstanding (shares)		91,530,000	103,403,000	104,881,000
SL Green Operating Partnership
Rollforward analysis of the activity relating to the noncontrolling interests in the operating partnership
Balance at beginning of period | $		$ 461,954,000	$ 473,882,000
Distributions | $		(15,000,000)	(14,266,000)
Issuance of common units | $		23,655,000	25,723,000
Redemption of common units | $		(60,718,000)	(21,574,000)
Net income | $		12,216,000	3,995,000
Accumulated other comprehensive income allocation | $		(66,000)	(94,000)
Fair value adjustment | $		(34,236,000)	(5,712,000)
Balance at end of period | $		$ 387,805,000	$ 461,954,000	$ 473,882,000
SL Green Operating Partnership | Series G Preferred Units
Rollforward analysis of the activity relating to the noncontrolling interests in the operating partnership
Number of preferred units issued (in shares)	1,902,000
Dividend rate preferred units (as a percent)	4.50%
Liquidation preference of preferred units (in dollars per share) | $ / shares	$ 25.00
Annual dividends on preferred units (in dollars per share) | $ / shares		$ 1.125
Operating partnership common stock value use for conversion of preferred units (in dollars per share) | $ / shares		$ 88.50
Number of company common stock issue on redemption of operation partnership common units		1
SL Green Operating Partnership | Series F Preferred Units
Rollforward analysis of the activity relating to the noncontrolling interests in the operating partnership
Number of preferred units issued (in shares)		60
Mandatory liquidation preference (in dollars per share) | $		$ 1,000.00
SL Green Operating Partnership | Series K Preferred Units
Rollforward analysis of the activity relating to the noncontrolling interests in the operating partnership
Number of preferred units issued (in shares)										563,954
Dividend rate preferred units (as a percent)		3.50%
Liquidation preference of preferred units (in dollars per share) | $ / shares		$ 25.00
Annual dividends on preferred units (in dollars per share) | $ / shares		0.875
Operating partnership common stock value use for conversion of preferred units (in dollars per share) | $ / shares		$ 134.67
Preferred Units, shares authorized		700,000
SL Green Operating Partnership | Series L Preferred Units
Rollforward analysis of the activity relating to the noncontrolling interests in the operating partnership
Number of preferred units issued (in shares)										378,634
Dividend rate preferred units (as a percent)		4.00%
Liquidation preference of preferred units (in dollars per share) | $ / shares		$ 25.00
Annual dividends on preferred units (in dollars per share) | $ / shares		$ 1.00
Preferred Units, shares authorized		500,000
SL Green Operating Partnership | Series M Preferred Units
Rollforward analysis of the activity relating to the noncontrolling interests in the operating partnership
Number of preferred units issued (in shares)									1,600,000
Dividend rate preferred units (as a percent)		3.75%
Liquidation preference of preferred units (in dollars per share) | $ / shares		$ 25.00
Annual dividends on preferred units (in dollars per share) | $ / shares		$ 0.9375
Preferred Units, shares authorized		1,600,000
SL Green Operating Partnership | Series N Preferred Units
Rollforward analysis of the activity relating to the noncontrolling interests in the operating partnership
Number of preferred units issued (in shares)								552,303
Dividend rate preferred units (as a percent)		3.00%
Liquidation preference of preferred units (in dollars per share) | $ / shares		$ 25.00
Annual dividends on preferred units (in dollars per share) | $ / shares		$ 0.75
Preferred Units, shares authorized		552,303
SL Green Operating Partnership | Series O Preferred Units
Rollforward analysis of the activity relating to the noncontrolling interests in the operating partnership
Dividend rate preferred units (as a percent)		6.25%
Preferred Units, shares authorized		1
SL Green Operating Partnership | Series P Preferred Units
Organization
Shares of common stock reserved for issuance upon redemption of units of limited partnership interest in operating partnership (shares)		200,000
Rollforward analysis of the activity relating to the noncontrolling interests in the operating partnership
Number of preferred units issued (in shares)							200,000
Dividend rate preferred units (as a percent)		4.00%
Liquidation preference of preferred units (in dollars per share) | $ / shares		$ 25.00
Annual dividends on preferred units (in dollars per share) | $ / shares		$ 1.00
SL Green Operating Partnership | Series Q Preferred Units
Organization
Shares of common stock reserved for issuance upon redemption of units of limited partnership interest in operating partnership (shares)		268,000
Rollforward analysis of the activity relating to the noncontrolling interests in the operating partnership
Number of preferred units issued (in shares)							268,000
Dividend rate preferred units (as a percent)		3.50%
Liquidation preference of preferred units (in dollars per share) | $ / shares		$ 25.00
Annual dividends on preferred units (in dollars per share) | $ / shares		0.875
Operating partnership common stock value use for conversion of preferred units (in dollars per share) | $ / shares		$ 148.95
SL Green Operating Partnership | Series R Preferred Units
Organization
Shares of common stock reserved for issuance upon redemption of units of limited partnership interest in operating partnership (shares)		400,000
Rollforward analysis of the activity relating to the noncontrolling interests in the operating partnership
Number of preferred units issued (in shares)						400,000
Dividend rate preferred units (as a percent)		3.50%
Liquidation preference of preferred units (in dollars per share) | $ / shares		$ 25.00
Annual dividends on preferred units (in dollars per share) | $ / shares		0.875
Operating partnership common stock value use for conversion of preferred units (in dollars per share) | $ / shares		$ 154.89
SL Green Operating Partnership | Series S Preferred Units
Organization
Shares of common stock reserved for issuance upon redemption of units of limited partnership interest in operating partnership (shares)		1,077,280
Rollforward analysis of the activity relating to the noncontrolling interests in the operating partnership
Number of preferred units issued (in shares)						1,077,280
Dividend rate preferred units (as a percent)		4.00%
Liquidation preference of preferred units (in dollars per share) | $ / shares		$ 25.00
Annual dividends on preferred units (in dollars per share) | $ / shares		$ 1.00
SL Green Operating Partnership | Series T Preferred Stock
Organization
Shares of common stock reserved for issuance upon redemption of units of limited partnership interest in operating partnership (shares)		230,000
Rollforward analysis of the activity relating to the noncontrolling interests in the operating partnership
Number of preferred units issued (in shares)					230,000
Dividend rate preferred units (as a percent)		2.75%
Liquidation preference of preferred units (in dollars per share) | $ / shares		$ 25.00
Annual dividends on preferred units (in dollars per share) | $ / shares		0.6875
Operating partnership common stock value use for conversion of preferred units (in dollars per share) | $ / shares		$ 119.02
SL Green Operating Partnership | Series U Preferred Stock
Organization
Shares of common stock reserved for issuance upon redemption of units of limited partnership interest in operating partnership (shares)		680,000
Rollforward analysis of the activity relating to the noncontrolling interests in the operating partnership
Number of preferred units issued (in shares)					680,000
Dividend rate preferred units (as a percent)		4.50%
Liquidation preference of preferred units (in dollars per share) | $ / shares		$ 25.00
Annual dividends on preferred units (in dollars per share) | $ / shares		$ 1.125
SL Green Operating Partnership | Series A Preferred Units
Organization
Shares of common stock reserved for issuance upon redemption of units of limited partnership interest in operating partnership (shares)		109,161
Rollforward analysis of the activity relating to the noncontrolling interests in the operating partnership
Number of preferred units issued (in shares)						109,161
Dividend rate preferred units (as a percent)		3.50%
Liquidation preference of preferred units (in dollars per share) | $ / shares		$ 1,000.00
Annual dividends on preferred units (in dollars per share) | $ / shares		$ 35.00
Number of company common stock issue on redemption of operation partnership common units		1
SL Green Operating Partnership | Series B Preferred Units
Rollforward analysis of the activity relating to the noncontrolling interests in the operating partnership
Number of preferred units issued (in shares)		0
SL Green Operating Partnership
Organization
Basic weighted average common shares outstanding (in shares)		91,315,000	103,127,000	104,508,000
Noncontrolling interest in the operating partnership (as a percent)		4.70%	4.58%
Number of units of operating partnership owned by the noncontrolling interest unit holders (shares)		4,130,579	4,452,979
Shares of common stock reserved for issuance upon redemption of units of limited partnership interest in operating partnership (shares)		4,130,579.000
Rollforward analysis of the activity relating to the noncontrolling interests in the operating partnership
Distributions | $		$ (8,364,000)	$ (52,446,000)	$ (15,419,000)
Accumulated other comprehensive income allocation | $		$ (66,000)	$ (94,000)	$ 1,299,000
Liquidation preference of preferred units (in dollars per share) | $ / shares		$ 25.00	$ 25.00
Stock-based compensation plans (shares)		215,000	276,000	373,000
Diluted weighted average common stock outstanding (shares)		91,530,000	103,403,000	104,881,000
Minimum | SL Green Operating Partnership | Series U Preferred Stock
Rollforward analysis of the activity relating to the noncontrolling interests in the operating partnership
Annual dividends on preferred units (in dollars per share) | $ / shares		$ 0.75