Investments in Unconsolidated Joint Ventures - Additional Information (Details) $ in Thousands	1 Months Ended						3 Months Ended	12 Months Ended
	Feb. 22, 2018	Nov. 30, 2017	Oct. 31, 2017	May 31, 2017 USD ($)	Oct. 31, 2016 ft²	Aug. 31, 2016 ft²	Dec. 31, 2017 USD ($) ft² unit	Dec. 31, 2017 USD ($) ft² unit	Sep. 30, 2017 USD ($)	Jan. 31, 2017 partner	Dec. 31, 2016 USD ($)
General information on each joint venture
Net equity investment in VIEs							$ 606,200	$ 606,200			$ 220,100
Approximate Square Feet (sqft) | ft²							33,253,213	33,253,213
11 Madison Avenue
General information on each joint venture
Approximate Square Feet (sqft) | ft²						2,314,000
Ownership percentage in disposed asset						40.00%
400 East 57th Street
General information on each joint venture
Approximate Square Feet (sqft) | ft²					290,482
Ownership percentage in disposed asset					49.00%
100 Park Avenue
General information on each joint venture
Ownership Interest							49.90%	49.90%
Economic Interest, percent							49.90%	49.90%
Approximate Square Feet (sqft) | ft²							834,000	834,000
Acquisition Price							$ 95,800	$ 95,800
717 Fifth Avenue
General information on each joint venture
Ownership Interest							10.92%	10.92%
Approximate Square Feet (sqft) | ft²							119,500	119,500
Acquisition Price							$ 251,900	$ 251,900
800 Third Avenue
General information on each joint venture
Ownership Interest							60.52%	60.52%
Economic Interest, percent							60.52%	60.52%
Approximate Square Feet (sqft) | ft²							526,000	526,000
Acquisition Price							$ 285,000	$ 285,000
1745 Broadway
General information on each joint venture
Ownership Interest							56.87%	56.87%
Economic Interest, percent							56.87%	56.87%
Approximate Square Feet (sqft) | ft²							674,000	674,000
Acquisition Price							$ 520,000	$ 520,000
Jericho Plaza
General information on each joint venture
Ownership Interest							11.67%	11.67%
Economic Interest, percent							11.67%	11.67%
Approximate Square Feet (sqft) | ft²							640,000	640,000
Acquisition Price							$ 210,000	$ 210,000
Investment in joint venture, investment expense							$ 6,600
11 West 34th Street
General information on each joint venture
Ownership Interest							30.00%	30.00%
Economic Interest, percent							30.00%	30.00%
Approximate Square Feet (sqft) | ft²							17,150	17,150
Acquisition Price							$ 10,800	$ 10,800
3 Columbus Circle
General information on each joint venture
Ownership Interest							48.90%	48.90%
Economic Interest, percent							48.90%	48.90%
Approximate Square Feet (sqft) | ft²							741,500	741,500
Acquisition Price							$ 500,000	$ 500,000
280 Park Avenue
General information on each joint venture
Ownership Interest							50.00%	50.00%
Economic Interest, percent							50.00%	50.00%
Approximate Square Feet (sqft) | ft²							1,219,158	1,219,158
Acquisition Price							$ 400,000	$ 400,000
1552-1560 Broadway
General information on each joint venture
Ownership Interest							50.00%	50.00%
Economic Interest, percent							50.00%	50.00%
Approximate Square Feet (sqft) | ft²							57,718	57,718
Acquisition Price							$ 136,550	$ 136,550
724 Fifth Avenue
General information on each joint venture
Ownership Interest							50.00%	50.00%
Economic Interest, percent							50.00%	50.00%
Approximate Square Feet (sqft) | ft²							65,010	65,010
Acquisition Price							$ 223,000	$ 223,000
10 East 53rd Street
General information on each joint venture
Ownership Interest							55.00%	55.00%
Economic Interest, percent							55.00%	55.00%
Approximate Square Feet (sqft) | ft²							354,300	354,300
Acquisition Price							$ 252,500	$ 252,500
521 Fifth Avenue
General information on each joint venture
Ownership Interest							50.50%	50.50%
Economic Interest, percent							50.50%	50.50%
Approximate Square Feet (sqft) | ft²							460,000	460,000
Acquisition Price							$ 315,000	$ 315,000
21 East 66th Street
General information on each joint venture
Ownership Interest							32.28%	32.28%
Economic Interest, percent							32.28%	32.28%
Approximate Square Feet (sqft) | ft²							13,069	13,069
Acquisition Price							$ 75,000	$ 75,000
21 East 66th Street | Three Retail and Two Residential Units
General information on each joint venture
Ownership Interest							32.28%	32.28%
Number of Stores | unit							3	3
Number of residential units | unit							2	2
21 East 66th Street | Three Residential Units
General information on each joint venture
Ownership Interest							16.14%	16.14%
Number of residential units | unit							3	3
650 Fifth Avenue
General information on each joint venture
Ownership Interest							50.00%	50.00%
Approximate Square Feet (sqft) | ft²							69,214	69,214
Acquisition Price							$ 0	$ 0
121 Greene Street
General information on each joint venture
Ownership Interest							50.00%	50.00%
Economic Interest, percent							50.00%	50.00%
Approximate Square Feet (sqft) | ft²							7,131	7,131
Acquisition Price							$ 27,400	$ 27,400
175-225 Third Street Brooklyn, New York
General information on each joint venture
Ownership Interest							95.00%	95.00%
Economic Interest, percent							95.00%	95.00%
Approximate Square Feet (sqft) | ft²							0	0
Acquisition Price							$ 74,600	$ 74,600
55 West 46th Street
General information on each joint venture
Ownership Interest							25.00%	25.00%
Economic Interest, percent							25.00%	25.00%
Approximate Square Feet (sqft) | ft²							347,000	347,000
Acquisition Price							$ 295,000	$ 295,000
Stonehenge Portfolio
General information on each joint venture
Ownership Interest					90.00%
Approximate Square Feet (sqft) | ft²							1,439,016	1,439,016
Acquisition Price							$ 36,668	$ 36,668
131-137 Spring Street
General information on each joint venture
Ownership Interest							20.00%	20.00%
Economic Interest, percent							20.00%	20.00%
Approximate Square Feet (sqft) | ft²							68,342	68,342
Acquisition Price							$ 277,750	$ 277,750
605 West 42nd Street
General information on each joint venture
Ownership Interest							20.00%	20.00%
Economic Interest, percent							20.00%	20.00%
Approximate Square Feet (sqft) | ft²							927,358	927,358
Acquisition Price							$ 759,000	$ 759,000
11 Madison Avenue
General information on each joint venture
Ownership Interest							60.00%	60.00%
Economic Interest, percent							60.00%	60.00%
Approximate Square Feet (sqft) | ft²							2,314,000	2,314,000
Acquisition Price							$ 2,605,000	$ 2,605,000
333 East 22nd St
General information on each joint venture
Ownership Interest							33.33%	33.33%
Economic Interest, percent							33.33%	33.33%
Approximate Square Feet (sqft) | ft²							26,926	26,926
Acquisition Price							$ 0	$ 0
400 East 57th Street
General information on each joint venture
Ownership Interest							51.00%	51.00%
Economic Interest, percent							41.00%	41.00%
Approximate Square Feet (sqft) | ft²							290,482	290,482
Acquisition Price							$ 170,000	$ 170,000
One Vanderbilt
General information on each joint venture
Ownership Interest							71.01%	71.01%
Economic Interest, percent							71.01%	71.01%
Approximate Square Feet (sqft) | ft²							0	0
Acquisition Price							$ 3,310,000	$ 3,310,000
Number of joint venture partners | partner										2
Investment in joint venture, aggregate equity committed by partner									$ 525,000
Investment in joint venture, aggregate maximum ownership percentage									29.00%
Investment in joint venture, partners' ownership percentage									3.48%
Worldwide Plaza
General information on each joint venture
Ownership Interest							24.35%	24.35%
Economic Interest, percent							24.35%	24.35%
Approximate Square Feet (sqft) | ft²							2,048,725	2,048,725
Acquisition Price							$ 1,725,000	$ 1,725,000
1515 Broadway
General information on each joint venture
Ownership Interest							69.87%	69.87%
Economic Interest, percent							69.87%	69.87%
Approximate Square Feet (sqft) | ft²							1,750,000	1,750,000
Acquisition Price							$ 1,950,000	$ 1,950,000
Equity method investments, depreciation								$ 900
1552 Broadway
General information on each joint venture
Approximate Square Feet (sqft) | ft²							13,045	13,045
Joint venture | Stonehenge Portfolio
General information on each joint venture
Ownership Interest					10.00%
Mezzanine Loan Contributed To Joint Venture In May 2017
General information on each joint venture
Ownership Interest				33.33%			33.33%	33.33%
Economic Interest, percent							33.33%	33.33%
Approximate Square Feet (sqft) | ft²							0	0
Acquisition Price							$ 15,000	$ 15,000
Equity method investment, amount sold				$ 10,000
RXR Realty | Joint venture | Worldwide Plaza
General information on each joint venture
Ownership Interest			50.00%
Ownership interest acquired (percent)			48.70%
New York REIT | Worldwide Plaza
General information on each joint venture
Ownership Interest			51.30%
Allianz Real Estate | 1515 Broadway
General information on each joint venture
Ownership percentage in disposed asset		30.13%
Subsequent Event | Allianz Real Estate | 1515 Broadway
General information on each joint venture
Ownership percentage in disposed asset	12.87%