Investments in Unconsolidated Joint Ventures - Additional Information (Details) $ in Thousands	1 Months Ended	3 Months Ended		6 Months Ended
	Oct. 31, 2016	Jun. 30, 2020 USD ($) ft² unit	Jun. 30, 2019 USD ($)	Jun. 30, 2020 USD ($) ft² unit	Jun. 30, 2019 USD ($)	Dec. 31, 2019 USD ($)
General information on each joint venture
Equity method investments | $		$ 2,952,681		$ 2,952,681		$ 2,912,842
Net equity investment in VIEs in which the entity is not primary beneficiary | $		$ 138,100		$ 138,100		145,900
Unaudited Approximate Square Feet (sqft)		29,843,409		29,843,409
Purchase price and other fair value adjustments | $		$ 0	$ 67,631	$ 0	$ 65,590
1552-1560 Broadway
General information on each joint venture
Unaudited Approximate Square Feet (sqft)		13,045		13,045
Stonehenge Portfolio
General information on each joint venture
Ownership Interest	90.00%
East 400 Street 57
General information on each joint venture
Ownership Interest	51.00%
885 Third Avenue
General information on each joint venture
Mortgage amount | $		$ 272,000		$ 272,000
Interest rate, fixed rate debt (as a percent)		3.35%		3.35%
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		1		1
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 | $		$ 2,952,681		$ 2,952,681		$ 2,912,842
Equity method investments with negative book value | $		$ 83,600		$ 83,600
Joint venture | 100 Park Avenue
General information on each joint venture
Ownership Interest		49.90%		49.90%
Economic Interest (as a percent)		49.90%		49.90%
Unaudited Approximate Square Feet (sqft)		834,000		834,000
Joint venture | 717 Fifth Avenue
General information on each joint venture
Ownership Interest		10.92%		10.92%
Economic Interest (as a percent)		10.92%		10.92%
Unaudited Approximate Square Feet (sqft)		119,500		119,500
Joint venture | 800 Third Avenue
General information on each joint venture
Ownership Interest		60.52%		60.52%
Economic Interest (as a percent)		60.52%		60.52%
Unaudited Approximate Square Feet (sqft)		526,000		526,000
Interest rate, fixed rate debt (as a percent)		3.37%		3.37%
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
Interest rate, fixed rate debt (as a percent)		5.12%		5.12%
Joint venture | 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
Joint venture | 280 Park Avenue
General information on each joint venture
Ownership Interest		50.00%		50.00%
Economic Interest (as a percent)		50.00%		50.00%
Unaudited Approximate Square Feet (sqft)		1,219,158		1,219,158
Joint venture | 1552-1560 Broadway
General information on each joint venture
Ownership Interest		50.00%		50.00%
Economic Interest (as a percent)		50.00%		50.00%
Unaudited Approximate Square Feet (sqft)		57,718		57,718
Joint venture | 10 East 53rd Street
General information on each joint venture
Ownership Interest		55.00%		55.00%
Economic Interest (as a percent)		55.00%		55.00%
Unaudited Approximate Square Feet (sqft)		354,300		354,300
Joint venture | 21 East 66th Street
General information on each joint venture
Ownership Interest		32.28%		32.28%
Economic Interest (as a percent)		32.28%		32.28%
Unaudited Approximate Square Feet (sqft)		13,069		13,069
Interest rate, fixed rate debt (as a percent)		3.60%		3.60%
Joint venture | 650 Fifth Avenue
General information on each joint venture
Ownership Interest		50.00%		50.00%
Economic Interest (as a percent)		50.00%		50.00%
Unaudited Approximate Square Feet (sqft)		69,214		69,214
Joint venture | 121 Greene Street
General information on each joint venture
Ownership Interest		50.00%		50.00%
Economic Interest (as a percent)		50.00%		50.00%
Unaudited Approximate Square Feet (sqft)		7,131		7,131
Joint venture | 55 West 46th Street
General information on each joint venture
Ownership Interest		25.00%		25.00%
Economic Interest (as a percent)		25.00%		25.00%
Unaudited Approximate Square Feet (sqft)		347,000		347,000
Joint venture | Stonehenge Portfolio
General information on each joint venture
Ownership Interest	10.00%
Unaudited Approximate Square Feet (sqft)		1,439,016		1,439,016
Interest rate, fixed rate debt (as a percent)		3.50%		3.50%
Joint venture | 605 West 42nd Street
General information on each joint venture
Ownership Interest		20.00%		20.00%
Economic Interest (as a percent)		20.00%		20.00%
Unaudited Approximate Square Feet (sqft)		927,358		927,358
Joint venture | 11 Madison Avenue
General information on each joint venture
Ownership Interest		60.00%		60.00%
Economic Interest (as a percent)		60.00%		60.00%
Unaudited Approximate Square Feet (sqft)		2,314,000		2,314,000
Interest rate, fixed rate debt (as a percent)		3.84%		3.84%
Joint venture | 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
Joint venture | East 400 Street 57
General information on each joint venture
Ownership Interest		51.00%		51.00%
Economic Interest (as a percent)		41.00%		41.00%
Unaudited Approximate Square Feet (sqft)		290,482		290,482
Interest rate, fixed rate debt (as a percent)		3.00%		3.00%
Joint venture | One Vanderbilt
General information on each joint venture
Ownership Interest		71.01%		71.01%
Economic Interest (as a percent)		71.01%		71.01%
Unaudited Approximate Square Feet (sqft)		0		0
Joint venture | 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
Interest rate, fixed rate debt (as a percent)		3.98%		3.98%
Joint venture | 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
Interest rate, fixed rate debt (as a percent)		3.93%		3.93%
Joint venture | 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
Joint venture | 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