Investments in Unconsolidated Joint Ventures (Details) $ in Thousands	6 Months Ended
	Jun. 30, 2016 USD ($) ft² unit note	Apr. 30, 2016	Dec. 31, 2015 USD ($)	Jul. 31, 2014 USD ($)
General information on each joint venture
Net equity investment in VIEs in which the entity is not primary beneficiary | $	$ 216,900		$ 39,700
Approximate Square Feet (sqft)	33,018,905
Mezzanine Loan Repaid in April 2016
General information on each joint venture
Ownership Interest (as a percent)		20.00%
Mezzanine loan | $				$ 50,000
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)	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)	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)	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)	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)	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)	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)	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)	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)	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)	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)	354,300
Acquisition Price | $	$ 252,500
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)	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)	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)	32,324
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)	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)	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)	347,000
Acquisition Price | $	$ 295,000
Stonehenge Portfolio
General information on each joint venture
Approximate Square Feet (sqft)	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)	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)	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%
605 West 42nd 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)	927,358
Acquisition Price | $	$ 759,000
1552 Broadway
General information on each joint venture
Approximate Square Feet (sqft)	13,045
Partners' Interest | 76 11th Avenue | Joint venture
General information on each joint venture
Exit fee, percentage owned	25.00%