Investments in Unconsolidated Joint Ventures - Additional Information (Details) $ in Thousands		1 Months Ended			2 Months Ended	3 Months Ended		9 Months Ended
	Jan. 01, 2018 USD ($)	Feb. 28, 2018	Nov. 30, 2017	Oct. 31, 2016	Mar. 31, 2018 property	Sep. 30, 2018 USD ($) ft² unit	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($) ft² unit	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)
General information on each joint venture
Equity method investments						$ 3,070,825		$ 3,070,825		$ 2,362,989
Net equity investment in VIEs in which the entity is not primary beneficiary						$ 722,400		$ 722,400		606,200
Unaudited Approximate Square Feet (sqft) | ft²						30,584,045		30,584,045
Purchase price and other fair value adjustments						$ (3,057)	$ 0	$ 57,385	$ 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
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						17.10%		17.10%
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
Allianz Real Estate | 1515 Broadway
General information on each joint venture
Ownership percentage in disposed asset		13.00%	30.00%
Joint venture
General information on each joint venture
Equity method investments						$ 3,070,825		$ 3,070,825		$ 2,362,989
Equity method investments with negative book value						$ 81,900		$ 81,900
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 | 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
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 | 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%