Investments in Unconsolidated Joint Ventures - Mortgages and Other Loans Payable (Details)	6 Months Ended
	Jun. 30, 2021 USD ($) mortgage extension partner	Dec. 31, 2020 USD ($)
Debt Instrument [Line Items]
Total fixed rate debt	$ 1,078,690,000	$ 1,083,683,000
Total floating rate debt	795,902,000	917,677,000
Revolving credit facility, net	0	105,262,000
Joint venture
Debt Instrument [Line Items]
Total fixed rate debt	8,323,168,000	5,604,858,000
Total floating rate debt	2,401,254,000	4,257,792,000
Total fixed rate and floating rate debt	10,724,422,000	9,862,650,000
Deferred financing costs, net	(143,318,000)	(113,446,000)
Total joint venture mortgages and other loans payable, net	$ 10,581,104,000	9,749,204,000
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	$ 654,000	677,000
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	$ 811,275,000	820,607,000
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	$ 96,666,000	97,024,000
Joint venture | Worldwide Plaza
Debt Instrument [Line Items]
Economic Interest (as a percent)	24.95%
Interest rate, fixed rate debt (as a percent)	3.98%
Total fixed rate debt	$ 1,200,000,000	1,200,000,000
Joint venture | One Vanderbilt
Debt Instrument [Line Items]
Economic Interest (as a percent)	71.01%
Interest rate, fixed rate debt (as a percent)	2.95%
Total fixed rate debt	$ 3,000,000,000	0
Total floating rate debt	$ 0	1,210,329,000
Joint venture | Stonehenge Portfolio
Debt Instrument [Line Items]
Interest rate, fixed rate debt (as a percent)	3.50%
Total fixed rate debt	$ 195,899,000	195,899,000
Joint venture | 885 Third Avenue
Debt Instrument [Line Items]
Interest rate, fixed rate debt (as a percent)
Total fixed rate debt	$ 0	272,000,000
Joint venture | 121 Greene Street
Debt Instrument [Line Items]
Economic Interest (as a percent)	50.00%
Interest rate, floating rate debt (as a percent)	2.00%
Total floating rate debt	$ 13,577,000	15,000,000
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
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	214,500,000
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 | 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 | 100 Park Avenue
Debt Instrument [Line Items]
Economic Interest (as a percent)	49.90%
Interest rate, floating rate debt (as a percent)	2.25%
Total floating rate debt	$ 360,000,000	360,000,000
Joint venture | 15 Beekman
Debt Instrument [Line Items]
Economic Interest (as a percent)	20.00%
Interest rate, floating rate debt (as a percent)	1.50%
Total floating rate debt	$ 27,002,000	11,212,000
Joint venture | 15 Beekman | Construction Loans
Debt Instrument [Line Items]
Maximum facility capacity	$ 125,000,000.0
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	220,000,000
Joint venture | One Madison Avenue
Debt Instrument [Line Items]
Economic Interest (as a percent)	50.50%
Interest rate, floating rate debt (as a percent)	3.35%
Total floating rate debt	$ 81,971,000	0
Joint venture | One Madison Avenue | Construction Loans
Debt Instrument [Line Items]
Maximum facility capacity	$ 1,250,000,000
Term (in Years)	5 years
Number of extension options | extension	1
Period of extension options	1 year
Joint venture | 605 West 42nd Street
Debt Instrument [Line Items]
Interest rate, floating rate debt (as a percent)
Total floating rate debt	$ 0	550,000,000
Joint venture | 55 West 46th Street
Debt Instrument [Line Items]
Interest rate, floating rate debt (as a percent)
Total floating rate debt	$ 0	$ 192,524,000
Secured Debt, Initial Maturity April 2028 | Joint venture | Stonehenge Portfolio
Debt Instrument [Line Items]
Total fixed rate debt	$ 132,400,000
Number of mortgages | partner	3
Secured Debt, Initial Maturity July 2029 | Joint venture | Stonehenge Portfolio
Debt Instrument [Line Items]
Total fixed rate debt	$ 63,500,000
Number of mortgages | mortgage	2