Investments in Unconsolidated Joint Ventures (Details) $ in Thousands	1 Months Ended
	May 31, 2017 USD ($)	Apr. 30, 2017 ft²	Oct. 31, 2016	Jun. 30, 2017 USD ($) ft² unit	Jan. 31, 2017 partner	Dec. 31, 2016 USD ($)
General information on each joint venture
Net equity investment in VIEs in which the entity is not primary beneficiary | $				$ 628,500		$ 220,100
Unaudited Approximate Square Feet (sqft) | ft²				32,134,288
100 Park Avenue
General information on each joint venture
Ownership Interest (as a percent)				49.90%
Economic Interest (as a percent)				49.90%
Unaudited 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%
Unaudited 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%
Unaudited Approximate Square Feet (sqft) | ft²				526,000
Acquisition Price | $				$ 285,000
1745 Broadway
General information on each joint venture
Ownership Interest (as a percent)				56.87%
Economic Interest (as a percent)				56.87%
Unaudited 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%
Unaudited 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%
Unaudited 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%
Unaudited 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%
Unaudited 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%
Unaudited 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%
Unaudited 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%
Unaudited 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%
Unaudited 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%
Unaudited 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%
Unaudited 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%
Unaudited 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%
Unaudited 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%
Unaudited Approximate Square Feet (sqft) | ft²				347,000
Acquisition Price | $				$ 295,000
Stonehenge Portfolio
General information on each joint venture
Ownership Interest (as a percent)			90.00%
Unaudited 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)			10.00%
131-137 Spring Street
General information on each joint venture
Ownership Interest (as a percent)				20.00%
Economic Interest (as a percent)				20.00%
Unaudited Approximate Square Feet (sqft) | ft²				68,342
Acquisition Price | $				$ 277,750
605 West 42nd Street
General information on each joint venture
Ownership Interest (as a percent)				20.00%
Economic Interest (as a percent)				20.00%
Unaudited Approximate Square Feet (sqft) | ft²				927,358
Acquisition Price | $				$ 759,000
11 Madison Avenue
General information on each joint venture
Ownership Interest (as a percent)				60.00%
Economic Interest (as a percent)				60.00%
Unaudited 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%
Unaudited Approximate Square Feet (sqft) | ft²				26,926
Acquisition Price | $				$ 0
East 400 Street 57
General information on each joint venture
Ownership Interest (as a percent)			51.00%	51.00%
Economic Interest (as a percent)				41.00%
Unaudited Approximate Square Feet (sqft) | ft²				290,482
Acquisition Price | $				$ 170,000
1552 Broadway
General information on each joint venture
Unaudited Approximate Square Feet (sqft) | ft²				13,045
One Vanderbilt
General information on each joint venture
Ownership Interest (as a percent)				71.01%
Number of joint venture partners | partner					2
Economic Interest (as a percent)				71.01%
Unaudited Approximate Square Feet (sqft) | ft²				0
Acquisition Price | $				$ 3,310,000
Aggregate equity committed by partners | $				$ 525,000
Maximum ownership percentage of partners				29.00%
Partners' ownership percentage				3.50%
Equity method investment, amount sold | $	$ 10,000
102 Greene Street
General information on each joint venture
Ownership Interest (as a percent)				10.00%
Economic Interest (as a percent)				10.00%
Unaudited Approximate Square Feet (sqft) | ft²				9,200
Acquisition Price | $				$ 43,500
East 400 Street 57
General information on each joint venture
Ownership percentage in disposed asset			49.00%
102 Greene Street
General information on each joint venture
Unaudited Approximate Square Feet (sqft) | ft²		9,000
Joint venture, ownership percentage sold		90.00%
21 East 66th Street | Three Retail and Two Residential Units
General information on each joint venture
Ownership Interest (as a percent)				3228.00%
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)				1614.00%
Number of residential units | unit				3
Mezzanine Loan
General information on each joint venture
Ownership Interest (as a percent)	33.33%			33.33%
Economic Interest (as a percent)				33.33%
Unaudited Approximate Square Feet (sqft) | ft²				0
Acquisition Price | $				$ 15,000