Noncontrolling Interests on the Company's Consolidated Financial Statements - Additional Information (Details) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2020 USD ($) shares $ / shares	Sep. 30, 2019 USD ($)	Jun. 30, 2020 USD ($)	Jun. 30, 2019 USD ($)	Sep. 30, 2020 USD ($) shares $ / shares	Sep. 30, 2019 USD ($)	Dec. 31, 2019 USD ($) shares
Rollforward analysis of the activity relating to the noncontrolling interests in the operating partnership
Redemption and conversion of common units | $	$ 0	$ (54)	$ (1,536)	$ (25,791)
Net income | $	802	1,719			$ 10,073	$ 12,306
Accumulated other comprehensive loss allocation | $	317	(607)			(2,664)	(2,869)
Partnership Interest
Rollforward analysis of the activity relating to the noncontrolling interests in the operating partnership
Balance at beginning of period | $			409,862	387,805	409,862	387,805	$ 387,805
Distributions | $					(8,857)		(14,729)
Issuance of common units | $					7,976		19,403
Redemption and conversion of common units | $					(27,657)		(27,962)
Net income | $					10,073		13,301
Accumulated other comprehensive loss allocation | $					(2,664)		(2,276)
Fair value adjustment | $					(35,253)		34,320
Balance at end of period | $	$ 353,480				$ 353,480		$ 409,862
SL Green Operating Partnership
Organization
Number of units of operating partnership owned by the noncontrolling interest unit holders (shares)	4,027,317				4,027,317		4,195,875
Shares of common stock reserved for issuance upon redemption of units of limited partnership interest in operating partnership (shares)	4,027,317				4,027,317
Rollforward analysis of the activity relating to the noncontrolling interests in the operating partnership
Redemption and conversion of common units | $	$ 0	(54)	$ (1,536)	$ (25,791)
Accumulated other comprehensive loss allocation | $	$ 317	$ (607)			$ (2,664)	$ (2,869)
SL Green Operating Partnership | 3.50% Series A
Rollforward analysis of the activity relating to the noncontrolling interests in the operating partnership
Dividend rate preferred units (as a percent)					3.50%
Preferred units, shares authorized (in shares)	109,161				109,161
Number of preferred units issued (in shares)	109,161				109,161
Number of Units Outstanding (in shares)	109,161				109,161
Annual dividends on preferred units (in dollars per share) | $ / shares					$ 35.0000
Liquidation preference of preferred units (in dollars per share) | $ / shares	$ 1,000.00				1,000.00
Operating partnership common stock value use for conversion of preferred units (in dollars per share) | $ / shares	$ 0				$ 0
Number of company common stock issued on conversion of Series B preferred units					6.71348
SL Green Operating Partnership | 7.00% Series F
Rollforward analysis of the activity relating to the noncontrolling interests in the operating partnership
Dividend rate preferred units (as a percent)					7.00%
Preferred units, shares authorized (in shares)	60				60
Number of preferred units issued (in shares)	60				60
Number of Units Outstanding (in shares)	60				60
Annual dividends on preferred units (in dollars per share) | $ / shares					$ 70.0000
Liquidation preference of preferred units (in dollars per share) | $ / shares	$ 1,000.00				1,000.00
Operating partnership common stock value use for conversion of preferred units (in dollars per share) | $ / shares	$ 29.12				$ 29.12
SL Green Operating Partnership | 4.50% Series G
Rollforward analysis of the activity relating to the noncontrolling interests in the operating partnership
Dividend rate preferred units (as a percent)					4.50%
Preferred units, shares authorized (in shares)	1,902,000				1,902,000
Number of preferred units issued (in shares)	1,902,000				1,902,000
Number of Units Outstanding (in shares)	863,972				863,972
Annual dividends on preferred units (in dollars per share) | $ / shares					$ 1.1250
Liquidation preference of preferred units (in dollars per share) | $ / shares	$ 25.00				25.00
Operating partnership common stock value use for conversion of preferred units (in dollars per share) | $ / shares	$ 88.50				$ 88.50
Number of company common stock issue on redemption of operation partnership common units					1
SL Green Operating Partnership | 3.50% Series K
Rollforward analysis of the activity relating to the noncontrolling interests in the operating partnership
Dividend rate preferred units (as a percent)					3.50%
Preferred units, shares authorized (in shares)	700,000				700,000
Number of preferred units issued (in shares)	563,954				563,954
Number of Units Outstanding (in shares)	341,677				341,677
Annual dividends on preferred units (in dollars per share) | $ / shares					$ 0.8750
Liquidation preference of preferred units (in dollars per share) | $ / shares	$ 25.00				25.00
Operating partnership common stock value use for conversion of preferred units (in dollars per share) | $ / shares	$ 134.67				$ 134.67
SL Green Operating Partnership | 4.00% Series L
Rollforward analysis of the activity relating to the noncontrolling interests in the operating partnership
Dividend rate preferred units (as a percent)					4.00%
Preferred units, shares authorized (in shares)	500,000				500,000
Number of preferred units issued (in shares)	378,634				378,634
Number of Units Outstanding (in shares)	372,634				372,634
Annual dividends on preferred units (in dollars per share) | $ / shares					$ 1.0000
Liquidation preference of preferred units (in dollars per share) | $ / shares	$ 25.00				25.00
Operating partnership common stock value use for conversion of preferred units (in dollars per share) | $ / shares	$ 0				$ 0
SL Green Operating Partnership | 3.75% Series M
Rollforward analysis of the activity relating to the noncontrolling interests in the operating partnership
Dividend rate preferred units (as a percent)					3.75%
Preferred units, shares authorized (in shares)	1,600,000				1,600,000
Number of preferred units issued (in shares)	1,600,000				1,600,000
Number of Units Outstanding (in shares)	96,357				96,357
Annual dividends on preferred units (in dollars per share) | $ / shares					$ 0.9375
Liquidation preference of preferred units (in dollars per share) | $ / shares	$ 25.00				25.00
Operating partnership common stock value use for conversion of preferred units (in dollars per share) | $ / shares	$ 0				$ 0
SL Green Operating Partnership | 4.00% Series P
Rollforward analysis of the activity relating to the noncontrolling interests in the operating partnership
Dividend rate preferred units (as a percent)					4.00%
Preferred units, shares authorized (in shares)	200,000				200,000
Number of preferred units issued (in shares)	200,000				200,000
Number of Units Outstanding (in shares)	200,000				200,000
Annual dividends on preferred units (in dollars per share) | $ / shares					$ 1.0000
Liquidation preference of preferred units (in dollars per share) | $ / shares	$ 25.00				25.00
Operating partnership common stock value use for conversion of preferred units (in dollars per share) | $ / shares	$ 0				$ 0
SL Green Operating Partnership | 3.50% Series Q
Rollforward analysis of the activity relating to the noncontrolling interests in the operating partnership
Dividend rate preferred units (as a percent)					3.50%
Preferred units, shares authorized (in shares)	268,000				268,000
Number of preferred units issued (in shares)	268,000				268,000
Number of Units Outstanding (in shares)	268,000				268,000
Annual dividends on preferred units (in dollars per share) | $ / shares					$ 0.8750
Liquidation preference of preferred units (in dollars per share) | $ / shares	$ 25.00				25.00
Operating partnership common stock value use for conversion of preferred units (in dollars per share) | $ / shares	$ 148.95				$ 148.95
SL Green Operating Partnership | 3.50% Series R
Rollforward analysis of the activity relating to the noncontrolling interests in the operating partnership
Dividend rate preferred units (as a percent)					3.50%
Preferred units, shares authorized (in shares)	400,000				400,000
Number of preferred units issued (in shares)	400,000				400,000
Number of Units Outstanding (in shares)	400,000				400,000
Annual dividends on preferred units (in dollars per share) | $ / shares					$ 0.8750
Liquidation preference of preferred units (in dollars per share) | $ / shares	$ 25.00				25.00
Operating partnership common stock value use for conversion of preferred units (in dollars per share) | $ / shares	$ 154.89				$ 154.89
SL Green Operating Partnership | 4.00% Series S
Rollforward analysis of the activity relating to the noncontrolling interests in the operating partnership
Dividend rate preferred units (as a percent)					4.00%
Preferred units, shares authorized (in shares)	1,077,280				1,077,280
Number of preferred units issued (in shares)	1,077,280				1,077,280
Number of Units Outstanding (in shares)	1,077,280				1,077,280
Annual dividends on preferred units (in dollars per share) | $ / shares					$ 1.0000
Liquidation preference of preferred units (in dollars per share) | $ / shares	$ 25.00				25.00
Operating partnership common stock value use for conversion of preferred units (in dollars per share) | $ / shares	$ 0				$ 0
SL Green Operating Partnership | 3.50% Series V
Rollforward analysis of the activity relating to the noncontrolling interests in the operating partnership
Dividend rate preferred units (as a percent)					3.50%
Preferred units, shares authorized (in shares)	40,000				40,000
Number of preferred units issued (in shares)	40,000				40,000
Number of Units Outstanding (in shares)	40,000				40,000
Annual dividends on preferred units (in dollars per share) | $ / shares					$ 0.8750
Liquidation preference of preferred units (in dollars per share) | $ / shares	$ 25.00				25.00
Operating partnership common stock value use for conversion of preferred units (in dollars per share) | $ / shares	$ 0				$ 0
SL Green Operating Partnership | Series W
Rollforward analysis of the activity relating to the noncontrolling interests in the operating partnership
Preferred units, shares authorized (in shares)	1				1
Number of preferred units issued (in shares)	1				1
Number of Units Outstanding (in shares)	1				1
Operating partnership common stock value use for conversion of preferred units (in dollars per share) | $ / shares	$ 0				$ 0
Preferred stock, dividend, multiplier per common unit distribution					1,350
Operating Partnership | SL Green Operating Partnership
Organization
Noncontrolling interest in the operating partnership (as a percent)	5.23%				5.23%		5.03%