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, 2019 USD ($) shares $ / shares	Sep. 30, 2018 USD ($)	Jun. 30, 2019 USD ($)	Sep. 30, 2019 USD ($) shares $ / shares	Sep. 30, 2018 USD ($)	Dec. 31, 2018 USD ($) shares
Rollforward analysis of the activity relating to the noncontrolling interests in the operating partnership
Balance at beginning of period | $			$ 387,805	$ 387,805
Redemption of common units | $	$ (54)		(25,791)
Net income | $	1,719	$ 4,797		12,306	$ 15,656
Accumulated other comprehensive income allocation | $	(607)	210		(2,869)	1,075
Balance at end of period | $	$ 401,863			$ 401,863		$ 387,805
Number of company common stock issued on conversion of Series B preferred units				6.71348
Series B Preferred Unit
Rollforward analysis of the activity relating to the noncontrolling interests in the operating partnership
Preferred units, issued (in shares) | shares	0			0
SL Green Operating Partnership
Organization
Noncontrolling interest in the operating partnership (as a percent)	4.96%			4.96%		4.70%
Number of units of operating partnership owned by the noncontrolling interest unit holders (shares) | shares	4,257,754			4,257,754		4,130,579
Shares of common stock reserved for issuance upon redemption of units of limited partnership interest in operating partnership (shares) | shares	4,257,754			4,257,754
Rollforward analysis of the activity relating to the noncontrolling interests in the operating partnership
Balance at beginning of period | $			387,805	$ 387,805	461,954	$ 461,954
Distributions | $				(10,993)		(15,000)
Issuance of common units | $				16,493		23,655
Redemption of common units | $	$ (54)		$ (25,791)	(16,388)		(60,718)
Net income | $				12,306		12,216
Accumulated other comprehensive income allocation | $	(607)	$ 210		(2,869)	$ 1,075	(66)
Fair value adjustment | $				15,509		(34,236)
Balance at end of period | $	$ 401,863			$ 401,863		$ 387,805
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 | 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 | 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 | 3.00% Series N
Rollforward analysis of the activity relating to the noncontrolling interests in the operating partnership
Dividend rate preferred units (as a percent)				3.00%
Preferred units, shares authorized (in shares) | shares	552,303			552,303
Number of preferred units issued (in shares) | shares	552,303			552,303
Annual dividends on preferred units (in dollars per share)				$ 0.7500
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 O
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
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 | 2.75% Series T
Rollforward analysis of the activity relating to the noncontrolling interests in the operating partnership
Dividend rate preferred units (as a percent)				2.75%
Preferred units, shares authorized (in shares) | shares	230,000			230,000
Number of preferred units issued (in shares) | shares	230,000			230,000
Annual dividends on preferred units (in dollars per share)				$ 0.6875
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)	$ 119.02			$ 119.02
SL Green Operating Partnership | 4.50% Series U
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	680,000			680,000
Number of preferred units issued (in shares) | shares	680,000			680,000
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)	$ 0			$ 0
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
SL Green Operating Partnership | 3.50% Series A | Minimum
Rollforward analysis of the activity relating to the noncontrolling interests in the operating partnership
Dividend rate preferred units (as a percent)				75.00%
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