Investments in Unconsolidated Joint Ventures - Additional Information (Details) $ in Thousands	1 Months Ended		3 Months Ended		6 Months Ended
	Feb. 28, 2019 property	Oct. 31, 2016	Jun. 30, 2019 USD ($) ft² unit	Jun. 30, 2018 USD ($)	Jun. 30, 2019 USD ($) ft² unit	Jun. 30, 2018 USD ($)	Dec. 31, 2018 USD ($)
General information on each joint venture
Equity method investments			$ 2,937,153		$ 2,937,153		$ 3,019,020
Net equity investment in VIEs in which the entity is not primary beneficiary			$ 873,000		$ 873,000		808,300
Unaudited Approximate Square Feet (sqft) | ft²			29,540,006		29,540,006
Purchase price and other fair value adjustments			$ 67,631	$ 11,149	$ 65,590	$ 60,442
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
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
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	1
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
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
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) | ft²			369,000		369,000
Acquisition Price			$ 266,000		$ 266,000
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
Joint venture
General information on each joint venture
Equity method investments			$ 2,937,153		$ 2,937,153		$ 3,019,020
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 | 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%