Investments in Unconsolidated Joint Ventures - Additional Information (Details) $ in Thousands	1 Months Ended
	May 31, 2017 USD ($)	Oct. 31, 2016	Sep. 30, 2017 USD ($) ft² unit	Apr. 30, 2017 ft²	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 | $			$ 598,700			$ 220,100
Unaudited Approximate Square Feet (sqft) | ft²			31,800,088
100 Park Avenue
General information on each joint venture
Ownership Interest			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			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			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			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			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			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			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			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			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			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			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			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			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			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			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			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			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		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		10.00%
131-137 Spring Street
General information on each joint venture
Ownership Interest			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			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			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			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		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			71.01%
Economic Interest (as a percent)			71.01%
Unaudited Approximate Square Feet (sqft) | ft²			0
Acquisition Price | $			$ 3,310,000
Number of joint venture partners | partner					2
Aggregate equity committed by partners | $			$ 525,000
Maximum ownership percentage of partners			29.00%
Partners' ownership percentage			3.49%
Equity method investment, amount sold | $	$ 10,000
102 Greene Street
General information on each joint venture
Ownership Interest			10.00%
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				9,200
21 East 66th Street | Three Retail and Two Residential Units
General information on each joint venture
Ownership Interest			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			1614.00%
Number of residential units | unit			3
Mezzanine Loan
General information on each joint venture
Ownership Interest	33.33%		33.33%
Economic Interest (as a percent)			33.33%
Unaudited Approximate Square Feet (sqft) | ft²			0
Acquisition Price | $			$ 15,000