Investments in Unconsolidated Joint Ventures - Additional Information (Details) $ in Thousands	1 Months Ended	3 Months Ended								12 Months Ended
	Oct. 31, 2016	Dec. 31, 2019 USD ($) ft² unit	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2019 USD ($) ft² unit	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
General information on each joint venture
Investments in unconsolidated joint ventures | $		$ 2,912,842				$ 3,019,020				$ 2,912,842	$ 3,019,020
Net equity investment in VIEs | $		$ 145,900				808,300				$ 145,900	808,300
Approximate Square Feet		27,607,971								27,607,971
Purchase price and other fair value adjustment | $		$ 0	$ 3,799	$ 67,631	$ (2,041)	$ 0	$ (3,057)	$ 11,149	$ 49,293	$ 69,389	$ 57,385	$ 0
21 East 66th Street | Three Retail and Two Residential Units
General information on each joint venture
Ownership Interest Sold		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 Sold		16.14%								16.14%
Number of residential units | unit		3								3
Stonehenge Portfolio
General information on each joint venture
Ownership Interest Sold	90.00%
400 East 57th Street
General information on each joint venture
Ownership Interest Sold	51.00%
1552 Broadway
General information on each joint venture
Approximate Square Feet		13,045								13,045
Joint venture
General information on each joint venture
Investments in unconsolidated joint ventures | $		$ 2,912,842								$ 2,912,842
Equity method investments with negative book value | $		$ 80,900								$ 80,900
Joint venture | 100 Park Avenue
General information on each joint venture
Ownership Interest Sold		49.90%								49.90%
Economic Interest, percent		49.90%								49.90%
Approximate Square Feet		834,000								834,000
Joint venture | 717 Fifth Avenue
General information on each joint venture
Ownership Interest Sold		10.92%								10.92%
Economic Interest, percent		10.92%								10.92%
Approximate Square Feet		119,500								119,500
Joint venture | 800 Third Avenue
General information on each joint venture
Ownership Interest Sold		60.52%								60.52%
Economic Interest, percent		60.52%								60.52%
Approximate Square Feet		526,000								526,000
Joint venture | 919 Third Avenue
General information on each joint venture
Ownership Interest Sold		51.00%								51.00%
Economic Interest, percent		51.00%								51.00%
Approximate Square Feet		1,454,000								1,454,000
Joint venture | 11 West 34th Street
General information on each joint venture
Ownership Interest Sold		30.00%								30.00%
Economic Interest, percent		30.00%								30.00%
Approximate Square Feet		17,150								17,150
Joint venture | 280 Park Avenue
General information on each joint venture
Ownership Interest Sold		50.00%								50.00%
Economic Interest, percent		50.00%								50.00%
Approximate Square Feet		1,219,158								1,219,158
Joint venture | 1552 Broadway
General information on each joint venture
Ownership Interest Sold		50.00%								50.00%
Economic Interest, percent		50.00%								50.00%
Approximate Square Feet		57,718								57,718
Joint venture | 10 East 53rd Street
General information on each joint venture
Ownership Interest Sold		55.00%								55.00%
Economic Interest, percent		55.00%								55.00%
Approximate Square Feet		354,300								354,300
Joint venture | 21 East 66th Street
General information on each joint venture
Ownership Interest Sold		32.28%								32.28%
Economic Interest, percent		32.28%								32.28%
Approximate Square Feet		13,069								13,069
Joint venture | 650 Fifth Avenue
General information on each joint venture
Ownership Interest Sold		50.00%								50.00%
Economic Interest, percent		50.00%								50.00%
Approximate Square Feet		69,214								69,214
Joint venture | 121 Greene Street
General information on each joint venture
Ownership Interest Sold		50.00%								50.00%
Economic Interest, percent		50.00%								50.00%
Approximate Square Feet		7,131								7,131
Joint venture | 55 West 46th Street
General information on each joint venture
Ownership Interest Sold		25.00%								25.00%
Economic Interest, percent		25.00%								25.00%
Approximate Square Feet		347,000								347,000
Joint venture | Stonehenge Portfolio
General information on each joint venture
Ownership Interest Sold	10.00%
Approximate Square Feet		1,439,016								1,439,016
Joint venture | 605 West 42nd Street
General information on each joint venture
Ownership Interest Sold		20.00%								20.00%
Economic Interest, percent		20.00%								20.00%
Approximate Square Feet		927,358								927,358
Joint venture | 11 Madison Avenue
General information on each joint venture
Ownership Interest Sold		60.00%								60.00%
Economic Interest, percent		60.00%								60.00%
Approximate Square Feet		2,314,000								2,314,000
Joint venture | 333 East 22nd St
General information on each joint venture
Ownership Interest Sold		33.33%								33.33%
Economic Interest, percent		33.33%								33.33%
Approximate Square Feet		26,926								26,926
Joint venture | 400 East 57th Street
General information on each joint venture
Ownership Interest Sold		51.00%								51.00%
Economic Interest, percent		41.00%								41.00%
Approximate Square Feet		290,482								290,482
Joint venture | One Vanderbilt
General information on each joint venture
Ownership Interest Sold		71.01%								71.01%
Economic Interest, percent		71.01%								71.01%
Approximate Square Feet		0								0
Joint venture | Worldwide Plaza
General information on each joint venture
Ownership Interest Sold		24.35%								24.35%
Economic Interest, percent		24.35%								24.35%
Approximate Square Feet		2,048,725								2,048,725
Joint venture | 1515 Broadway
General information on each joint venture
Ownership Interest Sold		56.87%								56.87%
Economic Interest, percent		56.87%								56.87%
Approximate Square Feet		1,750,000								1,750,000
Joint venture | 2 Herald Square
General information on each joint venture
Ownership Interest Sold		51.00%								51.00%
Economic Interest, percent		51.00%								51.00%
Approximate Square Feet		369,000								369,000
Joint venture | Spring Street 115
General information on each joint venture
Ownership Interest Sold		51.00%								51.00%
Economic Interest, percent		51.00%								51.00%
Approximate Square Feet		5,218								5,218
919 Third Avenue | Joint venture | 919 Third Avenue
General information on each joint venture
Purchase price and other fair value adjustment | $										$ 49,300
400 East 57th Street
General information on each joint venture
Ownership percentage in disposed asset	49.00%