Noncontrolling Interests on the Company's Consolidated Financial Statements - Additional Information (Details) $ / shares in Units, $ in Thousands	3 Months Ended			6 Months Ended		12 Months Ended
	Jun. 30, 2020 USD ($) shares $ / shares	Mar. 31, 2020 USD ($)	Jun. 30, 2019 USD ($)	Jun. 30, 2020 USD ($) shares $ / shares	Jun. 30, 2019 USD ($)	Dec. 31, 2019 USD ($) shares
Rollforward analysis of the activity relating to the noncontrolling interests in the operating partnership
Balance at beginning of period | $		$ 409,862		$ 409,862
Redemption and conversion of common units | $	$ 0	(1,536)	$ (25,791)
Net income | $	3,070		8,310	9,272	$ 10,587
Accumulated other comprehensive loss allocation | $	(95)		(1,258)	(2,981)	(2,261)
Balance at end of period | $	358,702			358,702		$ 409,862
Partnership Interest
Rollforward analysis of the activity relating to the noncontrolling interests in the operating partnership
Balance at beginning of period | $		409,862		409,862	387,805	387,805
Distributions | $				(5,110)		(14,729)
Issuance of common units | $				5,613		19,403
Redemption and conversion of common units | $				(26,810)		(27,962)
Net income | $				9,272		13,301
Accumulated other comprehensive loss allocation | $				(2,981)		(2,276)
Fair value adjustment | $				(31,144)		34,320
Balance at end of period | $	$ 358,702			$ 358,702		$ 409,862
SL Green Operating Partnership
Organization
Number of units of operating partnership owned by the noncontrolling interest unit holders (shares) | shares	4,044,555			4,044,555		4,195,875
Shares of common stock reserved for issuance upon redemption of units of limited partnership interest in operating partnership (shares) | shares	4,044,555			4,044,555
Rollforward analysis of the activity relating to the noncontrolling interests in the operating partnership
Redemption and conversion of common units | $	$ 0	$ (1,536)	(25,791)
Accumulated other comprehensive loss allocation | $	$ (95)		$ (1,258)	$ (2,981)	$ (2,261)
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) | shares	109,161			109,161
Number of preferred units issued (in shares) | shares	109,161			109,161
Annual dividends on preferred units (in dollars per share)				$ 35.0000
Liquidation preference of preferred units (in dollars per share)	$ 1,000.00			1,000.00
Operating partnership common stock value use for conversion of preferred units (in dollars per share)	$ 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) | shares	60			60
Number of preferred units issued (in shares) | shares	60			60
Annual dividends on preferred units (in dollars per share)				$ 70.0000
Liquidation preference of preferred units (in dollars per share)	$ 1,000.00			1,000.00
Operating partnership common stock value use for conversion of preferred units (in dollars per share)	$ 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) | shares	1,902,000			1,902,000
Number of preferred units issued (in shares) | shares	1,902,000			1,902,000
Annual dividends on preferred units (in dollars per share)				$ 1.1250
Liquidation preference of preferred units (in dollars per share)	$ 25.00			25.00
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 | 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) | shares	700,000			700,000
Number of preferred units issued (in shares) | shares	563,954			563,954
Annual dividends on preferred units (in dollars per share)				$ 0.8750
Liquidation preference of preferred units (in dollars per share)	$ 25.00			25.00
Operating partnership common stock value use for conversion of preferred units (in dollars per share)	$ 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) | shares	500,000			500,000
Number of preferred units issued (in shares) | shares	378,634			378,634
Annual dividends on preferred units (in dollars per share)				$ 1.0000
Liquidation preference of preferred units (in dollars per share)	$ 25.00			25.00
Operating partnership common stock value use for conversion of preferred units (in dollars per share)	$ 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) | shares	1,600,000			1,600,000
Number of preferred units issued (in shares) | shares	1,600,000			1,600,000
Annual dividends on preferred units (in dollars per share)				$ 0.9375
Liquidation preference of preferred units (in dollars per share)	$ 25.00			25.00
Operating partnership common stock value use for conversion of preferred units (in dollars per share)	$ 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) | shares	200,000			200,000
Number of preferred units issued (in shares) | shares	200,000			200,000
Annual dividends on preferred units (in dollars per share)				$ 1.0000
Liquidation preference of preferred units (in dollars per share)	$ 25.00			25.00
Operating partnership common stock value use for conversion of preferred units (in dollars per share)	$ 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) | shares	268,000			268,000
Number of preferred units issued (in shares) | shares	268,000			268,000
Annual dividends on preferred units (in dollars per share)				$ 0.8750
Liquidation preference of preferred units (in dollars per share)	$ 25.00			25.00
Operating partnership common stock value use for conversion of preferred units (in dollars per share)	$ 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) | shares	400,000			400,000
Number of preferred units issued (in shares) | shares	400,000			400,000
Annual dividends on preferred units (in dollars per share)				$ 0.8750
Liquidation preference of preferred units (in dollars per share)	$ 25.00			25.00
Operating partnership common stock value use for conversion of preferred units (in dollars per share)	$ 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) | shares	1,077,280			1,077,280
Number of preferred units issued (in shares) | shares	1,077,280			1,077,280
Annual dividends on preferred units (in dollars per share)				$ 1.0000
Liquidation preference of preferred units (in dollars per share)	$ 25.00			25.00
Operating partnership common stock value use for conversion of preferred units (in dollars per share)	$ 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) | shares	40,000			40,000
Number of preferred units issued (in shares) | shares	40,000			40,000
Annual dividends on preferred units (in dollars per share)				$ 0.8750
Liquidation preference of preferred units (in dollars per share)	$ 25.00			25.00
Operating partnership common stock value use for conversion of preferred units (in dollars per share)	$ 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) | shares	1			1
Number of preferred units issued (in shares) | shares	1			1
Operating partnership common stock value use for conversion of preferred units (in dollars per share)	$ 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)	5.20%			5.20%		5.03%