Investments in Unconsolidated Joint Ventures (Details) $ in Thousands	1 Months Ended					3 Months Ended
	Oct. 31, 2015 USD ($)	Aug. 31, 2015 USD ($) ft²	Jul. 31, 2015 USD ($)	Mar. 31, 2015 USD ($)	Feb. 28, 2015 USD ($) property	Mar. 31, 2016 USD ($)	Dec. 31, 2015 USD ($) ft² unit	Dec. 31, 2014 USD ($)
General information on each joint venture
Net equity investment in VIEs in which the entity is not primary beneficiary							$ 99,600	$ 146,200
Approximate Square Feet (sqft) | ft²							35,581,060
131-137 Spring Street
General information on each joint venture
Approximate Square Feet (sqft) | ft²		68,342
Agreed sale consideration		$ 277,800
Ownership percentage in disposed asset		80.00%
Series M Preferred Units
General information on each joint venture
Preferred Units (as a percent)					3.75%
100 Park Avenue
General information on each joint venture
Ownership Percentage							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 Percentage							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 Percentage							60.52%
Economic Interest (as a percent)							60.52%
Approximate Square Feet (sqft) | ft²							526,000
Acquisition Price							$ 285,000
Additional Ownership Percentage Acquired (less than for the 1.0%)				17.56%
Additional Cost for additional interest in joint venture				$ 67,500
1745 Broadway
General information on each joint venture
Ownership Percentage							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 Percentage							77.78%
Economic Interest (as a percent)							77.78%
Approximate Square Feet (sqft) | ft²							640,000
Acquisition Price							$ 210,000
Additional Ownership Percentage Acquired (less than for the 1.0%)	57.52%
11 West 34th Street
General information on each joint venture
Ownership Percentage							30.00%
Economic Interest (as a percent)							30.00%
Approximate Square Feet (sqft) | ft²							17,150
Acquisition Price							$ 10,800
7 Renaissance
General information on each joint venture
Ownership Percentage							50.00%
Economic Interest (as a percent)							50.00%
Approximate Square Feet (sqft) | ft²							65,641
Acquisition Price							$ 4,000
7 Renaissance | Subsequent Event
General information on each joint venture
Agreed sale consideration						$ 20,700
3 Columbus Circle
General information on each joint venture
Ownership Percentage							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 Percentage							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 Percentage							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 Percentage							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 Percentage							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 Percentage							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 Percentage							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 Percentage							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 Percentage							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 Percentage							16.14%
Number Of Residential Units | unit							3
650 Fifth Avenue
General information on each joint venture
Ownership Percentage							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 Percentage							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 Percentage							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 Percentage							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					$ 40,200		$ 36,668
Additional Ownership Percentage Acquired (less than for the 1.0%)			1.00%
Additional Cost for additional interest in joint venture			$ 1,100
Increase in real estate investment ownership interest					$ 3,500
Number of Joint Venture Properties with an Increased Ownership Interest | property					6
Stonehenge Portfolio | Series M Preferred Units
General information on each joint venture
Issuance of aggregate liquidation preference of preferred units					$ 40,000
131-137 Spring Street
General information on each joint venture
Ownership Percentage							20.00%
Economic Interest (as a percent)							20.00%
Approximate Square Feet (sqft) | ft²							68,342
Acquisition Price							$ 277,750
1552 Broadway
General information on each joint venture
Approximate Square Feet (sqft) | ft²							13,045