Noncontrolling Interests on the Company's Consolidated Financial Statements (Details)	1 Months Ended		3 Months Ended		9 Months Ended		12 Months Ended
	Feb. 28, 2015 shares	Jan. 31, 2012 $ / shares shares	Sep. 30, 2015 USD ($) unit $ / shares shares	Sep. 30, 2014 USD ($)	Sep. 30, 2015 USD ($) unit $ / shares shares	Sep. 30, 2014 USD ($)	Dec. 31, 2014 USD ($) unit	Aug. 31, 2015 shares	Jul. 31, 2015 shares	Jun. 30, 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 | $					$ 469,524,000
Distributions | $					(114,828,000)
Redemption of common units | $					(20,650,000)
Net income | $			$ 6,467,000	$ 2,636,000	6,634,000	$ 16,010,000
Accumulated other comprehensive income allocation | $			(191,000)	$ 113,000	(349,000)	347,000
Balance at end of period | $			$ 423,421,000		$ 423,421,000		$ 469,524,000
Number of company common stock issued on conversion of Series B preferred units					6.71348
Series M Preferred Units
Rollforward analysis of the activity relating to the noncontrolling interests in the operating partnership
Dividend rate preferred units (as a percent)	3.75%
SL Green Operating Partnership
Organization
Noncontrolling interest in the operating partnership (as a percent)			3.74%		3.74%		3.92%
Number of units of operating partnership owned by the noncontrolling interest unit holders (shares) | unit			3,874,750		3,874,750		3,973,016
Shares of common stock reserved for issuance upon redemption of units of limited partnership interest in operating partnership (shares) | shares			3,874,750		3,874,750
Rollforward analysis of the activity relating to the noncontrolling interests in the operating partnership
Balance at beginning of period | $					$ 469,524,000	$ 265,476,000	$ 265,476,000
Distributions | $					(7,017,000)		(7,849,000)
Issuance of common units | $					27,898,000		56,469,000
Redemption of common units | $					(41,605,000)		(31,653,000)
Net income | $					6,634,000		18,467,000
Accumulated other comprehensive income allocation | $					(349,000)		175,000
Fair value adjustment | $					(31,664,000)		168,439,000
Balance at end of period | $			$ 423,421,000		$ 423,421,000		$ 469,524,000
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
Dividend rate preferred units (as a percent)		4.50%
Liquidation preference of preferred units (in dollars per share)		$ 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 | 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 | 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 | 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 | 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 | 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 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