Investments in Unconsolidated Joint Ventures - Mortgages and Other Loans Payable (Details)		6 Months Ended
	Aug. 06, 2020 extension	Jun. 30, 2020 USD ($) extension	Dec. 31, 2019 USD ($)	Aug. 31, 2019 USD ($)
Debt Instrument [Line Items]
Total fixed rate debt		$ 1,127,377,000	$ 1,383,449,000
Total floating rate debt		1,259,915,000	828,434,000
Total fixed rate and floating rate debt		2,387,292,000	2,211,883,000
Joint venture
Debt Instrument [Line Items]
Total fixed rate debt		5,342,130,000	5,351,721,000
Total floating rate debt		4,001,153,000	3,691,686,000
Total fixed rate and floating rate debt		9,343,283,000	9,043,407,000
Deferred financing costs, net		(94,088,000)	(91,538,000)
Total joint venture mortgages and other loans payable, net		$ 9,249,195,000	8,951,869,000
Joint venture | 717 Fifth Avenue
Debt Instrument [Line Items]
Economic Interest (as a percent)		10.92%
Joint venture | 717 Fifth Avenue | Mortgage loan
Debt Instrument [Line Items]
Interest rate, fixed rate debt (as a percent)		4.45%
Total fixed rate debt		$ 300,000,000	300,000,000
Joint venture | 717 Fifth Avenue | Mezzanine loans
Debt Instrument [Line Items]
Interest rate, fixed rate debt (as a percent)		5.50%
Total fixed rate debt		$ 355,328,000	355,328,000
Joint venture | 650 Fifth Avenue
Debt Instrument [Line Items]
Economic Interest (as a percent)		50.00%
Joint venture | 650 Fifth Avenue | Mortgage loan
Debt Instrument [Line Items]
Interest rate, fixed rate debt (as a percent)		4.46%
Total fixed rate debt		$ 210,000,000	210,000,000
Joint venture | 650 Fifth Avenue | Mezzanine loans
Debt Instrument [Line Items]
Interest rate, fixed rate debt (as a percent)		5.45%
Total fixed rate debt		$ 65,000,000	65,000,000
Joint venture | 21 East 66th Street
Debt Instrument [Line Items]
Economic Interest (as a percent)		32.28%
Interest rate, fixed rate debt (as a percent)		3.60%
Interest rate, floating rate debt (as a percent)		2.75%
Total fixed rate debt		$ 12,000,000	12,000,000
Total floating rate debt		$ 699,000	712,000
Joint venture | 919 Third Avenue
Debt Instrument [Line Items]
Economic Interest (as a percent)		51.00%
Interest rate, fixed rate debt (as a percent)		5.12%
Total fixed rate debt		$ 500,000,000	500,000,000
Joint venture | 1515 Broadway
Debt Instrument [Line Items]
Economic Interest (as a percent)		56.87%
Interest rate, fixed rate debt (as a percent)		3.93%
Total fixed rate debt		$ 829,666,000	838,546,000
Joint venture | 11 Madison Avenue
Debt Instrument [Line Items]
Economic Interest (as a percent)		60.00%
Interest rate, fixed rate debt (as a percent)		3.84%
Total fixed rate debt		$ 1,400,000,000	1,400,000,000
Joint venture | 800 Third Avenue
Debt Instrument [Line Items]
Economic Interest (as a percent)		60.52%
Interest rate, fixed rate debt (as a percent)		3.37%
Total fixed rate debt		$ 177,000,000	177,000,000
Joint venture | East 400 Street 57
Debt Instrument [Line Items]
Economic Interest (as a percent)		41.00%
Interest rate, fixed rate debt (as a percent)		3.00%
Total fixed rate debt		$ 97,024,000	97,735,000
Joint venture | Worldwide Plaza
Debt Instrument [Line Items]
Economic Interest (as a percent)		24.35%
Interest rate, fixed rate debt (as a percent)		3.98%
Total fixed rate debt		$ 1,200,000,000	1,200,000,000
Joint venture | Stonehenge Portfolio
Debt Instrument [Line Items]
Interest rate, fixed rate debt (as a percent)		3.50%
Total fixed rate debt		$ 196,112,000	196,112,000
Joint venture | 280 Park Avenue
Debt Instrument [Line Items]
Economic Interest (as a percent)		50.00%
Interest rate, floating rate debt (as a percent)		1.73%
Total floating rate debt		$ 1,200,000,000	1,200,000,000
Joint venture | 1552 Broadway
Debt Instrument [Line Items]
Economic Interest (as a percent)		50.00%
Interest rate, floating rate debt (as a percent)		2.65%
Total floating rate debt		$ 195,000,000	195,000,000
Number of extension options | extension		2
Period of extension options		1 year
Joint venture | 121 Greene Street
Debt Instrument [Line Items]
Economic Interest (as a percent)		50.00%
Interest rate, floating rate debt (as a percent)		1.50%
Total floating rate debt		$ 15,000,000	15,000,000
Joint venture | 11 West 34th Street
Debt Instrument [Line Items]
Economic Interest (as a percent)		30.00%
Interest rate, floating rate debt (as a percent)		1.45%
Total floating rate debt		$ 23,000,000	23,000,000
Joint venture | 100 Park Avenue
Debt Instrument [Line Items]
Economic Interest (as a percent)		49.90%
Interest rate, floating rate debt (as a percent)		1.75%
Total floating rate debt		$ 355,066,000	356,972,000
Joint venture | One Vanderbilt
Debt Instrument [Line Items]
Economic Interest (as a percent)		71.01%
Interest rate, floating rate debt (as a percent)		2.50%
Total floating rate debt		$ 969,814,000	732,928,000
Joint venture | One Vanderbilt | Construction Loans
Debt Instrument [Line Items]
Maximum facility capacity		$ 1,750,000,000
Term (in Years)		5 years
Number of extension options | extension		2
Period of extension options		1 year
Joint venture | 2 Herald Square
Debt Instrument [Line Items]
Economic Interest (as a percent)		51.00%
Interest rate, floating rate debt (as a percent)		1.45%
Total floating rate debt		$ 214,500,000	190,000,000
Joint venture | 55 West 46th Street
Debt Instrument [Line Items]
Economic Interest (as a percent)		25.00%
Interest rate, floating rate debt (as a percent)		1.25%
Total floating rate debt		$ 192,524,000	192,524,000
Joint venture | 55 West 46th Street | Mortgage loan
Debt Instrument [Line Items]
Committed amount				$ 198,000,000.0
Unfunded Amount		$ 5,500,000
Joint venture | 115 Spring Street
Debt Instrument [Line Items]
Economic Interest (as a percent)		51.00%
Interest rate, floating rate debt (as a percent)		3.40%
Total floating rate debt		$ 65,550,000	65,550,000
Joint venture | 10 East 53rd Street
Debt Instrument [Line Items]
Economic Interest (as a percent)		55.00%
Interest rate, floating rate debt (as a percent)		1.35%
Total floating rate debt		$ 220,000,000	170,000,000
Joint venture | 605 West 42nd Street
Debt Instrument [Line Items]
Economic Interest (as a percent)		20.00%
Interest rate, floating rate debt (as a percent)		1.44%
Total floating rate debt		$ 550,000,000	$ 550,000,000
Secured Debt, Initial Maturity August 2019 | Joint venture | Stonehenge Portfolio
Debt Instrument [Line Items]
Total fixed rate debt		132,600,000
Secured Debt, Initial Maturity June 2024 | Joint venture | Stonehenge Portfolio
Debt Instrument [Line Items]
Total fixed rate debt		$ 63,500,000
Subsequent Event | Joint venture | 1552 Broadway
Debt Instrument [Line Items]
Number of extension options exercised | extension	1