Investments in Unconsolidated Joint Ventures - Additional Information (Details) $ in Thousands		1 Months Ended				2 Months Ended	3 Months Ended		6 Months Ended
	Jan. 01, 2018 USD ($)	Feb. 28, 2018	Nov. 30, 2017	May 31, 2017 USD ($)	Oct. 31, 2016	Mar. 31, 2018 property	Jun. 30, 2018 USD ($) ft² unit	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($) ft² unit	Jun. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)
General information on each joint venture
Net equity investment in VIEs in which the entity is not primary beneficiary							$ 680,100		$ 680,100		$ 606,200
Unaudited Approximate Square Feet (sqft) | ft²							31,150,063		31,150,063
Purchase price and other fair value adjustments							$ 11,149	$ 0	$ 60,442	$ 0
100 Park Avenue
General information on each joint venture
Ownership Interest							49.90%		49.90%
Unaudited 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%
Unaudited 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%
Unaudited Approximate Square Feet (sqft) | ft²							526,000		526,000
Acquisition Price							$ 285,000		$ 285,000
11 West 34th Street
General information on each joint venture
Ownership Interest							30.00%		30.00%
Unaudited 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%
Unaudited 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%
Unaudited 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%
Unaudited 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%
Unaudited 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%
Unaudited 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%
Unaudited 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%
Unaudited Approximate Square Feet (sqft) | ft²							13,069		13,069
Acquisition Price							$ 75,000		$ 75,000
650 Fifth Avenue
General information on each joint venture
Ownership Interest							50.00%		50.00%
Unaudited 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%
Unaudited Approximate Square Feet (sqft) | ft²							7,131		7,131
Acquisition Price							$ 27,400		$ 27,400
55 West 46th Street
General information on each joint venture
Ownership Interest							25.00%		25.00%
Unaudited 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%
Unaudited Approximate Square Feet (sqft) | ft²							1,439,016		1,439,016
Acquisition Price							$ 36,668		$ 36,668
Number of properties sold | property						2
131-137 Spring Street
General information on each joint venture
Ownership Interest							20.00%		20.00%
Unaudited 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%
Unaudited 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%
Unaudited 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 (as a percent)							33.33%		33.33%
Unaudited Approximate Square Feet (sqft) | ft²							26,926		26,926
Acquisition Price							$ 0		$ 0
East 400 Street 57
General information on each joint venture
Ownership Interest					51.00%		51.00%		51.00%
Unaudited Approximate Square Feet (sqft) | ft²							290,482		290,482
Acquisition Price							$ 170,000		$ 170,000
1552 Broadway
General information on each joint venture
Unaudited Approximate Square Feet (sqft) | ft²							13,045		13,045
One Vanderbilt
General information on each joint venture
Ownership Interest							71.01%		71.01%
Unaudited Approximate Square Feet (sqft) | ft²							0		0
Acquisition Price							$ 3,310,000		$ 3,310,000
Aggregate equity committed by partners							$ 525,000		$ 525,000
Maximum ownership percentage of partners							29.00%		29.00%
Partners' ownership percentage							13.39%		13.39%
Equity method investment, amount sold				$ 10,000
Worldwide Plaza
General information on each joint venture
Ownership Interest							24.35%		24.35%
Economic Interest (as a percent)							24.35%		24.35%
Unaudited 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							56.87%		56.87%
Economic Interest (as a percent)							56.87%		56.87%
Unaudited Approximate Square Feet (sqft) | ft²							1,750,000		1,750,000
Acquisition Price							$ 1,950,000		$ 1,950,000
Cumulative effect on retained earnings	$ 600,000
East 400 Street 57
General information on each joint venture
Ownership percentage in disposed asset					49.00%
21 East 66th Street | Three Retail and Two Residential Units
General information on each joint venture
Ownership Interest							3228.00%		3228.00%
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							1614.00%		1614.00%
Number of residential units | unit							3		3
Mezzanine Loan
General information on each joint venture
Ownership Interest				33.33%			33.33%		33.33%
Economic Interest (as a percent)							33.33%		33.33%
Unaudited Approximate Square Feet (sqft) | ft²							0		0
Acquisition Price							$ 15,000		$ 15,000
Allianz Real Estate | 1515 Broadway
General information on each joint venture
Ownership percentage in disposed asset		13.00%	30.00%
Joint venture | 100 Park Avenue
General information on each joint venture
Economic Interest (as a percent)							49.90%		49.90%
Joint venture | 717 Fifth Avenue
General information on each joint venture
Economic Interest (as a percent)							10.92%		10.92%
Joint venture | 800 Third Avenue
General information on each joint venture
Economic Interest (as a percent)							60.52%		60.52%
Joint venture | 11 West 34th Street
General information on each joint venture
Economic Interest (as a percent)							30.00%		30.00%
Joint venture | 3 Columbus Circle
General information on each joint venture
Economic Interest (as a percent)							48.90%		48.90%
Joint venture | 280 Park Avenue
General information on each joint venture
Economic Interest (as a percent)							50.00%		50.00%
Joint venture | 1552-1560 Broadway
General information on each joint venture
Economic Interest (as a percent)							50.00%		50.00%
Joint venture | 724 Fifth Avenue
General information on each joint venture
Economic Interest (as a percent)							50.00%		50.00%
Joint venture | 10 East 53rd Street
General information on each joint venture
Economic Interest (as a percent)							55.00%		55.00%
Joint venture | 521 Fifth Avenue
General information on each joint venture
Economic Interest (as a percent)							50.50%		50.50%
Joint venture | 21 East 66th Street
General information on each joint venture
Economic Interest (as a percent)							32.28%		32.28%
Joint venture | 650 Fifth Avenue
General information on each joint venture
Economic Interest (as a percent)							50.00%		50.00%
Joint venture | 121 Greene Street
General information on each joint venture
Economic Interest (as a percent)							50.00%		50.00%
Joint venture | 55 West 46th Street
General information on each joint venture
Economic Interest (as a percent)							25.00%		25.00%
Joint venture | Stonehenge Portfolio
General information on each joint venture
Ownership Interest					10.00%
Joint venture | 131-137 Spring Street
General information on each joint venture
Economic Interest (as a percent)							20.00%		20.00%
Joint venture | 605 West 42nd Street
General information on each joint venture
Economic Interest (as a percent)							20.00%		20.00%
Joint venture | 11 Madison Avenue
General information on each joint venture
Economic Interest (as a percent)							60.00%		60.00%
Joint venture | East 400 Street 57
General information on each joint venture
Economic Interest (as a percent)							41.00%		41.00%
Joint venture | One Vanderbilt
General information on each joint venture
Economic Interest (as a percent)							71.01%		71.01%
Joint venture | 919 Third Avenue
General information on each joint venture
Ownership Interest							51.00%		51.00%
Economic Interest (as a percent)							51.00%		51.00%
Unaudited Approximate Square Feet (sqft) | ft²							1,454,000		1,454,000
Acquisition Price							$ 1,256,727		$ 1,256,727