Investments in Unconsolidated Joint Ventures - Mortgages and Other Loans Payable (Details)	12 Months Ended
	Dec. 31, 2020 USD ($) extension	Dec. 31, 2019 USD ($)	Aug. 31, 2019 USD ($)
Debt Instrument [Line Items]
Total fixed rate debt	$ 1,083,683,000	$ 1,383,449,000
Total floating rate debt	917,677,000	828,434,000
Deferred financing costs, net	(21,388,000)	(28,630,000)
Joint venture
Debt Instrument [Line Items]
Total fixed rate debt	5,604,858,000	5,351,721,000
Total floating rate debt	4,257,792,000	3,691,686,000
Total fixed rate and floating rate debt	9,862,650,000	9,043,407,000
Deferred financing costs, net	(113,446,000)	(91,538,000)
Total joint venture mortgages and other loans payable, net	$ 9,749,204,000	8,951,869,000
Joint venture | 885 Third Avenue
Debt Instrument [Line Items]
Economic Interest, percent	100.00%
Joint venture | 885 Third Avenue | Mortgage loan
Debt Instrument [Line Items]
Interest rate, fixed rate debt (as a percent)	3.35%
Total fixed rate debt	$ 272,000,000	0
Joint venture | 717 Fifth Avenue
Debt Instrument [Line Items]
Economic Interest, 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, 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, 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	$ 677,000	712,000
Joint venture | 919 Third Avenue
Debt Instrument [Line Items]
Economic Interest, 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, percent	56.87%
Interest rate, fixed rate debt (as a percent)	3.93%
Total fixed rate debt	$ 820,607,000	838,546,000
Joint venture | 11 Madison Avenue
Debt Instrument [Line Items]
Economic Interest, 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, 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 | 400 East 57th Street
Debt Instrument [Line Items]
Economic Interest, 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, 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	$ 195,899,000	196,112,000
Joint venture | Stonehenge Portfolio | Initial Maturity August 2019
Debt Instrument [Line Items]
Total fixed rate debt	132,400,000
Joint venture | Stonehenge Portfolio | Initial Maturity June 2024
Debt Instrument [Line Items]
Total fixed rate debt	$ 63,500,000
Joint venture | 121 Greene Street
Debt Instrument [Line Items]
Economic Interest, 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 | 280 Park Avenue
Debt Instrument [Line Items]
Economic Interest, 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 | One Vanderbilt
Debt Instrument [Line Items]
Economic Interest, percent	71.01%
Interest rate, floating rate debt (as a percent)	2.50%
Total floating rate debt	$ 1,210,329,000	732,928,000
Joint venture | One Vanderbilt | Construction Loans
Debt Instrument [Line Items]
Maximum facility capacity	$ 1,750,000,000
Term (in Years)	3 years
Number of extension options | extension	2
Period of extension option	1 year
Joint venture | 1552 Broadway
Debt Instrument [Line Items]
Economic Interest, 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, 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 | 11 West 34th Street
Debt Instrument [Line Items]
Economic Interest, 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 | 55 West 46th Street (5)
Debt Instrument [Line Items]
Economic Interest, 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 (5) | Mortgage loan
Debt Instrument [Line Items]
Committed amount			$ 198,000,000.0
Debt instrument unfunded amount	$ 5,500,000
Joint venture | 115 Spring Street
Debt Instrument [Line Items]
Economic Interest, 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, percent	49.90%
Interest rate, floating rate debt (as a percent)	2.25%
Total floating rate debt	$ 360,000,000	356,972,000
Joint venture | 15 Beekman
Debt Instrument [Line Items]
Economic Interest, percent	20.00%
Interest rate, floating rate debt (as a percent)	1.50%
Total floating rate debt	$ 11,212,000	0
Joint venture | 10 East 53rd Street
Debt Instrument [Line Items]
Economic Interest, 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 | One Madison Avenue
Debt Instrument [Line Items]
Economic Interest, percent	50.50%
Interest rate, floating rate debt (as a percent)	3.35%
Total floating rate debt	$ 0	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 option	1 year
Joint venture | 605 West 42nd Street
Debt Instrument [Line Items]
Economic Interest, percent	20.00%
Interest rate, floating rate debt (as a percent)	1.44%
Total floating rate debt	$ 550,000,000	$ 550,000,000
Joint venture | 126 Nassau Street | Construction Loans
Debt Instrument [Line Items]
Maximum facility capacity	$ 125,000,000.0