Noncontrolling Interests on the Company's Consolidated Financial Statements - Additional Information (Details)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018 USD ($) shares $ / shares	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($) shares $ / shares	Jun. 30, 2017 USD ($)	Dec. 31, 2017 USD ($) shares	Jun. 30, 2016 shares	Aug. 31, 2015 shares	Jul. 31, 2015 shares	Jun. 30, 2015 shares	Feb. 28, 2015 shares	Aug. 31, 2014 shares
Rollforward analysis of the activity relating to the noncontrolling interests in the operating partnership
Balance at beginning of period | $			$ 461,954,000
Net income | $	$ 5,586,000	$ 419,000	10,858,000	$ 895,000
Accumulated other comprehensive income allocation | $	245,000	(100,000)	864,000	(299,000)
Balance at end of period | $	$ 486,610,000		$ 486,610,000		$ 461,954,000
Number of company common stock issued on conversion of Series B preferred units			6.71348
SL Green Operating Partnership
Organization
Noncontrolling interest in the operating partnership (as a percent)	5.20%		5.20%		4.58%
Number of units of operating partnership owned by the noncontrolling interest unit holders (shares) | shares	4,699,872		4,699,872		4,452,979
Shares of common stock reserved for issuance upon redemption of units of limited partnership interest in operating partnership (shares) | shares	4,699,872.000		4,699,872.000
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	$ 473,882,000
Distributions | $			(7,712,000)		(14,266,000)
Issuance of common units | $			17,714,000		25,723,000
Redemption of common units | $			(1,561,000)		(21,574,000)
Net income | $			10,858,000		3,995,000
Accumulated other comprehensive income allocation | $	$ 245,000	$ (100,000)	864,000	$ (299,000)	(94,000)
Fair value adjustment | $			4,493,000		(5,712,000)
Balance at end of period | $	$ 486,610,000		$ 486,610,000		$ 461,954,000
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) | shares	1,902,000		1,902,000
Dividend rate preferred units (as a percent)			4.50%
Liquidation preference of preferred units (in dollars per share)	$ 25.00		$ 25.00
Annual dividends on preferred units (in dollars per share)			1.125
Operating partnership common stock value use for conversion of preferred units (in dollars per share)	$ 88.50		$ 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) | shares	60		60
Mandatory liquidation preference (in dollars per share) | $	$ 1,000.00		$ 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) | shares											563,954
Dividend rate preferred units (as a percent)			3.50%
Liquidation preference of preferred units (in dollars per share)	$ 25.00		$ 25.00
Annual dividends on preferred units (in dollars per share)			0.875
Operating partnership common stock value use for conversion of preferred units (in dollars per share)	$ 134.67		$ 134.67
Preferred units, shares authorized (in shares) | shares	700,000		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) | shares											378,634
Dividend rate preferred units (as a percent)			4.00%
Liquidation preference of preferred units (in dollars per share)	$ 25.00		$ 25.00
Annual dividends on preferred units (in dollars per share)			$ 1.00
Preferred units, shares authorized (in shares) | shares	500,000		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) | shares										1,600,000
Dividend rate preferred units (as a percent)			3.75%
Liquidation preference of preferred units (in dollars per share)	$ 25.00		$ 25.00
Annual dividends on preferred units (in dollars per share)			$ 0.9375
Preferred units, shares authorized (in shares) | shares	1,600,000		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) | shares									552,303
Dividend rate preferred units (as a percent)			3.00%
Liquidation preference of preferred units (in dollars per share)	$ 25.00		$ 25.00
Annual dividends on preferred units (in dollars per share)			$ 0.75
Preferred units, shares authorized (in shares) | shares	552,303		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 (in shares) | shares	1		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) | shares	200,000		200,000
Rollforward analysis of the activity relating to the noncontrolling interests in the operating partnership
Number of preferred units issued (in shares) | shares								200,000
Dividend rate preferred units (as a percent)			4.00%
Liquidation preference of preferred units (in dollars per share)	$ 25.00		$ 25.00
Annual dividends on preferred units (in dollars per share)			$ 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) | shares	268,000		268,000
Rollforward analysis of the activity relating to the noncontrolling interests in the operating partnership
Number of preferred units issued (in shares) | shares								268,000
Dividend rate preferred units (as a percent)			3.50%
Liquidation preference of preferred units (in dollars per share)	$ 25.00		$ 25.00
Annual dividends on preferred units (in dollars per share)			0.875
Operating partnership common stock value use for conversion of preferred units (in dollars per share)	$ 148.95		$ 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) | shares	400,000		400,000
Rollforward analysis of the activity relating to the noncontrolling interests in the operating partnership
Number of preferred units issued (in shares) | shares							400,000
Dividend rate preferred units (as a percent)			3.50%
Liquidation preference of preferred units (in dollars per share)	$ 25.00		$ 25.00
Annual dividends on preferred units (in dollars per share)			0.875
Operating partnership common stock value use for conversion of preferred units (in dollars per share)	$ 154.89		$ 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) | shares	1,077,280		1,077,280
Rollforward analysis of the activity relating to the noncontrolling interests in the operating partnership
Number of preferred units issued (in shares) | shares							1,077,280
Dividend rate preferred units (as a percent)			4.00%
Liquidation preference of preferred units (in dollars per share)	$ 25.00		$ 25.00
Annual dividends on preferred units (in dollars per share)			$ 1.00
SL Green Operating Partnership | Series T Preferred Units
Organization
Shares of common stock reserved for issuance upon redemption of units of limited partnership interest in operating partnership (shares) | shares	230,000		230,000
Rollforward analysis of the activity relating to the noncontrolling interests in the operating partnership
Number of preferred units issued (in shares) | shares						230,000
Dividend rate preferred units (as a percent)			2.75%
Liquidation preference of preferred units (in dollars per share)	$ 25.00		$ 25.00
Annual dividends on preferred units (in dollars per share)			0.6875
Operating partnership common stock value use for conversion of preferred units (in dollars per share)	$ 119.02		$ 119.02
SL Green Operating Partnership | Series U Preferred Units
Organization
Shares of common stock reserved for issuance upon redemption of units of limited partnership interest in operating partnership (shares) | shares	680,000		680,000
Rollforward analysis of the activity relating to the noncontrolling interests in the operating partnership
Number of preferred units issued (in shares) | shares						680,000
Dividend rate preferred units (as a percent)			4.50%
Liquidation preference of preferred units (in dollars per share)	$ 25.00		$ 25.00
Annual dividends on preferred units (in dollars per share)			1.125
SL Green Operating Partnership | Series U Preferred Units | Minimum
Rollforward analysis of the activity relating to the noncontrolling interests in the operating partnership
Annual dividends on preferred units (in dollars per share)			$ 0.75
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) | shares	109,161		109,161
Rollforward analysis of the activity relating to the noncontrolling interests in the operating partnership
Number of preferred units issued (in shares) | shares							109,161
Dividend rate preferred units (as a percent)			3.50%
Liquidation preference of preferred units (in dollars per share)	$ 1,000.00		$ 1,000.00
Annual dividends on preferred units (in dollars per share)			$ 35.00
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) | shares	0		0