Investments in Unconsolidated Joint Ventures - Additional Information (Details) $ in Thousands	1 Months Ended		3 Months Ended		9 Months Ended
	Feb. 28, 2019 property	Oct. 31, 2016	Sep. 30, 2019 USD ($) ft² unit	Sep. 30, 2018 USD ($)	Sep. 30, 2019 USD ($) ft² unit	Sep. 30, 2018 USD ($)	Dec. 31, 2018 USD ($)
General information on each joint venture
Equity method investments | $			$ 2,923,595		$ 2,923,595		$ 3,019,020
Net equity investment in VIEs in which the entity is not primary beneficiary | $			$ 876,900		$ 876,900		808,300
Unaudited Approximate Square Feet (sqft)			29,540,006		29,540,006
Purchase price and other fair value adjustments | $			$ 3,799	$ (3,057)	$ 69,389	$ 57,385
100 Park Avenue
General information on each joint venture
Ownership Interest			49.90%		49.90%
Unaudited Approximate Square Feet (sqft)			834,000		834,000
717 Fifth Avenue
General information on each joint venture
Ownership Interest			10.92%		10.92%
Unaudited Approximate Square Feet (sqft)			119,500		119,500
800 Third Avenue
General information on each joint venture
Ownership Interest			60.52%		60.52%
Unaudited Approximate Square Feet (sqft)			526,000		526,000
11 West 34th Street
General information on each joint venture
Ownership Interest			30.00%		30.00%
Economic Interest (as a percent)			30.00%		30.00%
Unaudited Approximate Square Feet (sqft)			17,150		17,150
280 Park Avenue
General information on each joint venture
Ownership Interest			50.00%		50.00%
Unaudited Approximate Square Feet (sqft)			1,219,158		1,219,158
1552-1560 Broadway
General information on each joint venture
Ownership Interest			50.00%		50.00%
Unaudited Approximate Square Feet (sqft)			57,718		57,718
10 East 53rd Street
General information on each joint venture
Ownership Interest			55.00%		55.00%
Unaudited Approximate Square Feet (sqft)			354,300		354,300
21 East 66th Street
General information on each joint venture
Ownership Interest			32.28%		32.28%
Unaudited Approximate Square Feet (sqft)			13,069		13,069
650 Fifth Avenue
General information on each joint venture
Ownership Interest			50.00%		50.00%
Unaudited Approximate Square Feet (sqft)			69,214		69,214
121 Greene Street
General information on each joint venture
Ownership Interest			50.00%		50.00%
Unaudited Approximate Square Feet (sqft)			7,131		7,131
55 West 46th Street
General information on each joint venture
Ownership Interest			25.00%		25.00%
Unaudited Approximate Square Feet (sqft)			347,000		347,000
Stonehenge Portfolio
General information on each joint venture
Ownership Interest		90.00%
Unaudited Approximate Square Feet (sqft)			1,439,016		1,439,016
Number of properties sold | property	1
605 West 42nd Street
General information on each joint venture
Ownership Interest			20.00%		20.00%
Unaudited Approximate Square Feet (sqft)			927,358		927,358
11 Madison Avenue
General information on each joint venture
Ownership Interest			60.00%		60.00%
Unaudited Approximate Square Feet (sqft)			2,314,000		2,314,000
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)			26,926		26,926
East 400 Street 57
General information on each joint venture
Ownership Interest		51.00%	51.00%		51.00%
Unaudited Approximate Square Feet (sqft)			290,482		290,482
1552 Broadway
General information on each joint venture
Unaudited Approximate Square Feet (sqft)			13,045		13,045
One Vanderbilt
General information on each joint venture
Ownership Interest			71.01%		71.01%
Unaudited Approximate Square Feet (sqft)			0		0
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)			2,048,725		2,048,725
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)			1,750,000		1,750,000
2 Herald Square
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)			369,000		369,000
115 Spring Street
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)			5,218		5,218
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
Joint venture
General information on each joint venture
Equity method investments | $			$ 2,923,595		$ 2,923,595		$ 3,019,020
Equity method investments with negative book value | $			$ 82,100		$ 82,100
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)			1,454,000		1,454,000
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 | 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 | 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%