Investments in Unconsolidated Joint Ventures (Details) $ in Thousands	1 Months Ended	3 Months Ended	12 Months Ended
	Oct. 31, 2016	Mar. 31, 2017 USD ($) ft²	Dec. 31, 2016 USD ($) ft² unit
General information on each joint venture
Net equity investment in VIEs in which the entity is not primary beneficiary		$ 223,600	$ 220,100
Approximate Square Feet (sqft) | ft²		32,341,214
Joint venture
General information on each joint venture
Equity method investment, assets		$ 10,276,794	$ 10,712,595
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%
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		57,718
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		65,010
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
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		13,069
Acquisition Price		$ 75,000
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		69,214
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
Ownership Interest (as a percent)	9000.00%
Approximate Square Feet (sqft) | ft²		1,439,016
Acquisition Price		$ 36,668
Stonehenge Portfolio | Joint venture
General information on each joint venture
Ownership Interest (as a percent)	1000.00%
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)		35.09%
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 | unit			2
Exit fee, percentage owned			5000.00%
Extension Fee, Percentage Owned			3800.00%
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) | ft²		927,358
Acquisition Price		$ 759,000
1552 Broadway
General information on each joint venture
Approximate Square Feet (sqft) | ft²			13,045
11 Madison Avenue
General information on each joint venture
Ownership Interest (as a percent)		60.00%
Economic Interest (as a percent)		60.00%
Approximate Square Feet (sqft) | ft²		2,314,000
Acquisition Price		$ 2,605,000
333 East 22nd St
General information on each joint venture
Ownership Interest (as a percent)		33.33%
Economic Interest (as a percent)		33.33%
Approximate Square Feet (sqft) | ft²		26,926
Acquisition Price		$ 0
East 400 Street 57 [Member]
General information on each joint venture
Ownership Interest (as a percent)			51.00%
Economic Interest (as a percent)			41.00%
Approximate Square Feet (sqft) | ft²			290,482
Acquisition Price			$ 170,000
Partners' Interest | 76 11th Avenue | Joint venture
General information on each joint venture
Exit fee, percentage owned			2500.00%
Extension Fee, Percentage Owned			3100.00%
Partners' Interest | 11 Madison Avenue | Joint venture
General information on each joint venture
Equity method investments, depreciation		$ 29,500
East 400 Street 57 [Member]
General information on each joint venture
Ownership percentage in disposed asset	49.00%
Three Residential Units | 21 East 66th Street
General information on each joint venture
Ownership Interest (as a percent)			1614.00%
Number Of Residential Units | unit			3
Three Retail and Two Residential Units | 21 East 66th Street
General information on each joint venture
Ownership Interest (as a percent)			3228.00%
Number of Stores | unit			3
Number Of Residential Units | unit			2