Investments in Unconsolidated Joint Ventures (Details) $ in Thousands	1 Months Ended	3 Months Ended
	Oct. 31, 2015 USD ($)	Mar. 31, 2016 USD ($) ft² note unit	Dec. 31, 2015 USD ($)
General information on each joint venture
Net equity investment in VIEs in which the entity is not primary beneficiary | $		$ 181,300	$ 39,700
Approximate Square Feet (sqft) | ft²		34,850,908
100 Park Avenue
General information on each joint venture
Ownership Interest (as a percent)		49.90%
Economic Interest (as a percent)		49.90%
Approximate Square Feet (sqft) | ft²		834,000
Acquisition Price | $		$ 95,800
717 Fifth Avenue
General information on each joint venture
Ownership Interest (as a percent)		10.92%
Economic Interest (as a percent)		10.92%
Approximate Square Feet (sqft) | ft²		119,500
Acquisition Price | $		$ 251,900
800 Third Avenue
General information on each joint venture
Ownership Interest (as a percent)		60.52%
Economic Interest (as a percent)		60.52%
Approximate Square Feet (sqft) | ft²		526,000
Acquisition Price | $		$ 285,000
1745 Broadway
General information on each joint venture
Ownership Interest (as a percent)		56.88%
Economic Interest (as a percent)		56.88%
Approximate Square Feet (sqft) | ft²		674,000
Acquisition Price | $		$ 520,000
Jericho Plaza
General information on each joint venture
Ownership Interest (as a percent)		11.67%	77.78%
Economic Interest (as a percent)		11.67%
Approximate Square Feet (sqft) | ft²		640,000
Acquisition Price | $		$ 210,000
11 West 34th Street
General information on each joint venture
Ownership Interest (as a percent)		30.00%
Economic Interest (as a percent)		30.00%
Approximate Square Feet (sqft) | ft²		17,150
Acquisition Price | $		$ 10,800
3 Columbus Circle
General information on each joint venture
Ownership Interest (as a percent)		48.90%
Economic Interest (as a percent)		48.90%
Approximate Square Feet (sqft) | ft²		741,500
Acquisition Price | $		$ 500,000
280 Park Avenue
General information on each joint venture
Ownership Interest (as a percent)		50.00%
Economic Interest (as a percent)		50.00%
Approximate Square Feet (sqft) | ft²		1,219,158
Acquisition Price | $		$ 400,000
1552-1560 Broadway
General information on each joint venture
Ownership Interest (as a percent)		50.00%
Economic Interest (as a percent)		50.00%
Approximate Square Feet (sqft) | ft²		35,897
Acquisition Price | $		$ 136,550
724 Fifth Avenue
General information on each joint venture
Ownership Interest (as a percent)		50.00%
Economic Interest (as a percent)		50.00%
Approximate Square Feet (sqft) | ft²		65,040
Acquisition Price | $		$ 223,000
10 East 53rd Street
General information on each joint venture
Ownership Interest (as a percent)		55.00%
Economic Interest (as a percent)		55.00%
Approximate Square Feet (sqft) | ft²		354,300
Acquisition Price | $		$ 252,500
33 Beekman
General information on each joint venture
Ownership Interest (as a percent)		45.90%
Economic Interest (as a percent)		45.90%
Approximate Square Feet (sqft) | ft²		163,500
Acquisition Price | $		$ 31,000
Agreed sale consideration | $	$ 196,000
521 Fifth Avenue
General information on each joint venture
Ownership Interest (as a percent)		50.50%
Economic Interest (as a percent)		50.50%
Approximate Square Feet (sqft) | ft²		460,000
Acquisition Price | $		$ 315,000
21 East 66th Street
General information on each joint venture
Ownership Interest (as a percent)		32.28%
Economic Interest (as a percent)		32.28%
Approximate Square Feet (sqft) | ft²		16,736
Acquisition Price | $		$ 75,000
21 East 66th Street | Three Retail and Two Residential Units
General information on each joint venture
Ownership Interest (as a percent)		32.28%
Number of Stores | unit		3
Number Of Residential Units | unit		2
21 East 66th Street | Three Residential Units
General information on each joint venture
Ownership Interest (as a percent)		16.14%
Number Of Residential Units | unit		3
650 Fifth Avenue
General information on each joint venture
Ownership Interest (as a percent)		50.00%
Economic Interest (as a percent)		50.00%
Approximate Square Feet (sqft) | ft²		32,324
Acquisition Price | $		$ 0
121 Greene Street
General information on each joint venture
Ownership Interest (as a percent)		50.00%
Economic Interest (as a percent)		50.00%
Approximate Square Feet (sqft) | ft²		7,131
Acquisition Price | $		$ 27,400
175-225 Third Street Brooklyn, New York
General information on each joint venture
Ownership Interest (as a percent)		95.00%
Economic Interest (as a percent)		95.00%
Approximate Square Feet (sqft) | ft²		0
Acquisition Price | $		$ 74,600
55 West 46th Street
General information on each joint venture
Ownership Interest (as a percent)		25.00%
Economic Interest (as a percent)		25.00%
Approximate Square Feet (sqft) | ft²		347,000
Acquisition Price | $		$ 295,000
Stonehenge Portfolio
General information on each joint venture
Approximate Square Feet (sqft) | ft²		2,046,733
Acquisition Price | $		$ 36,668
131-137 Spring Street
General information on each joint venture
Ownership Interest (as a percent)		20.00%
Economic Interest (as a percent)		20.00%
Approximate Square Feet (sqft) | ft²		68,342
Acquisition Price | $		$ 277,750
76 11th Avenue
General information on each joint venture
Ownership Interest (as a percent)		33.33%
Economic Interest (as a percent)		36.58%
Approximate Square Feet (sqft) | ft²		764,000
Acquisition Price | $		$ 138,240
Number of mezzanine notes secured by interests in the entity which owns 76 11th avenue | note		2
Exit fee, percentage owned		50.00%
76 11th Avenue | Joint venture
General information on each joint venture
Exit fee, percentage owned		25.00%
1552 Broadway
General information on each joint venture
Approximate Square Feet (sqft) | ft²		13,045