Investments in Unconsolidated Joint Ventures - Additional Information (Details) $ in Thousands	1 Months Ended		3 Months Ended		9 Months Ended
	Aug. 31, 2020 USD ($)	Oct. 31, 2016	Sep. 30, 2020 USD ($) ft² unit	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($) ft² unit	Sep. 30, 2019 USD ($)	Dec. 31, 2019 USD ($)
General information on each joint venture
Equity method investments | $			$ 2,946,673		$ 2,946,673		$ 2,912,842
Net equity investment in VIEs in which the entity is not primary beneficiary | $			$ 136,300		$ 136,300		145,900
Unaudited Approximate Square Feet (sqft)			30,328,709		30,328,709
Purchase price and other fair value adjustments | $			$ 0	$ 3,799	$ 0	$ 69,389
Gain on sale of real estate, net | $			$ 26,104	$ 3,541	$ 163,624	$ 2,492
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%
126 Nassau Street
General information on each joint venture
Gain on sale of real estate, net | $	$ 17,700
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			32.28%		32.28%
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			16.14%		16.14%
Number of residential units | unit			3		3
Joint venture
General information on each joint venture
Equity method investments | $			$ 2,946,673		$ 2,946,673		$ 2,912,842
Equity method investments with negative book value | $			$ 85,800		$ 85,800
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
Joint venture | 885 Third Avenue
General information on each joint venture
Economic Interest (as a percent)			100.00%		100.00%
Unaudited Approximate Square Feet (sqft)			625,300		625,300
Interest rate, fixed rate debt (as a percent)			3.35%		3.35%
Joint venture | 126 Nassau 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)			98,412		98,412