Investments in Unconsolidated Joint Ventures - Mortgages and Other Loans Payable (Details)	1 Months Ended	9 Months Ended
	Feb. 28, 2019 property	Sep. 30, 2019 USD ($) extension	Aug. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Debt Instrument [Line Items]
Total fixed rate debt		$ 1,385,756,000		$ 1,389,717,000
Total floating rate debt		$ 1,068,927,000		598,442,000
1515 Broadway
Debt Instrument [Line Items]
Economic Interest (as a percent)		56.87%
Worldwide Plaza
Debt Instrument [Line Items]
Economic Interest (as a percent)		24.35%
Stonehenge Portfolio
Debt Instrument [Line Items]
Number of properties sold | property	1
11 West 34th Street
Debt Instrument [Line Items]
Economic Interest (as a percent)		30.00%
2 Herald Square
Debt Instrument [Line Items]
Economic Interest (as a percent)		51.00%
115 Spring Street
Debt Instrument [Line Items]
Economic Interest (as a percent)		51.00%
Total floating rate debt		$ 0		65,550,000
Joint venture
Debt Instrument [Line Items]
Total fixed rate debt		5,356,671,000		5,666,108,000
Total floating rate debt		3,539,744,000		3,387,705,000
Total fixed rate and floating rate debt		8,896,415,000		9,053,813,000
Deferred financing costs, net		(97,400,000)		(103,191,000)
Total joint venture mortgages and other loans payable, net		$ 8,799,015,000		8,950,622,000
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		$ 1,515,000		1,571,000
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]
Interest rate, fixed rate debt (as a percent)		3.93%
Total fixed rate debt		$ 842,966,000		855,876,000
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		$ 98,265,000		99,828,000
Joint venture | Worldwide Plaza
Debt Instrument [Line Items]
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		321,076,000
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		185,569,000
Joint venture | 55 West 46th Street | Mortgage loan
Debt Instrument [Line Items]
Interest rate, floating rate debt (as a percent)			1.25%
Face amount of loan			$ 192,500,000
Committed amount			198,000,000.0
Unfunded Amount			$ 5,500,000
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 | 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 | 10 East 53rd Street
Debt Instrument [Line Items]
Economic Interest (as a percent)		55.00%
Interest rate, floating rate debt (as a percent)		2.25%
Total floating rate debt		$ 170,000,000		170,000,000
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 | 11 West 34th Street
Debt Instrument [Line Items]
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		$ 357,897,000		360,000,000
Joint venture | One Vanderbilt
Debt Instrument [Line Items]
Economic Interest (as a percent)		71.01%
Interest rate, floating rate debt (as a percent)		2.75%
Total floating rate debt		$ 619,258,000		375,000,000
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]
Interest rate, floating rate debt (as a percent)		1.55%
Total floating rate debt		$ 150,000,000		133,565,000
Joint venture | 115 Spring Street
Debt Instrument [Line Items]
Interest rate, floating rate debt (as a percent)		3.40%
Total floating rate debt		$ 65,550,000		0
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
Joint venture | 131-137 Spring Street
Debt Instrument [Line Items]
Total floating rate debt		0		141,000,000
Joint venture | 103 East 86th Street
Debt Instrument [Line Items]
Total floating rate debt		0		38,000,000
Joint venture | 521 Fifth Avenue
Debt Instrument [Line Items]
Total fixed rate debt		0		$ 170,000,000
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