Investments in Unconsolidated Joint Ventures - Mortgages and Other Loans Payable (Details)	1 Months Ended		3 Months Ended
	Apr. 30, 2018	Mar. 31, 2018 USD ($)	Mar. 31, 2018 USD ($) extension	Dec. 31, 2017 USD ($)
Debt Instrument [Line Items]
Total fixed rate debt		$ 1,891,154,000	$ 1,891,154,000	$ 2,400,560,000
Total floating rate debt		565,026,000	565,026,000	465,431,000
Total fixed rate and floating rate debt		2,456,180,000	2,456,180,000	2,865,991,000
919 Third Avenue
Debt Instrument [Line Items]
Total fixed rate debt		$ 0	$ 0	500,000,000
1515 Broadway
Debt Instrument [Line Items]
Economic Interest (as a percent)		56.87%	56.87%
Worldwide Plaza
Debt Instrument [Line Items]
Economic Interest (as a percent)		24.35%	24.35%
Joint venture
Debt Instrument [Line Items]
Total fixed rate debt		$ 6,014,627,000	$ 6,014,627,000	5,569,138,000
Total floating rate debt		3,983,061,000	3,983,061,000	3,979,066,000
Total fixed rate and floating rate debt		9,997,688,000	9,997,688,000	9,548,204,000
Deferred financing costs, net		(120,809,000)	(120,809,000)	(136,103,000)
Total joint venture mortgages and other loans payable, net		$ 9,876,879,000	$ 9,876,879,000	9,412,101,000
Joint venture | 521 Fifth Avenue
Debt Instrument [Line Items]
Economic Interest (as a percent)		50.50%	50.50%
Interest rate, fixed rate debt (as a percent)		3.73%	3.73%
Total fixed rate debt		$ 170,000,000	$ 170,000,000	170,000,000
Joint venture | 717 Fifth Avenue
Debt Instrument [Line Items]
Economic Interest (as a percent)		10.92%	10.92%
Joint venture | 717 Fifth Avenue | Mortgage loan
Debt Instrument [Line Items]
Interest rate, fixed rate debt (as a percent)		4.45%	4.45%
Total fixed rate debt		$ 300,000,000	$ 300,000,000	300,000,000
Committed amount		$ 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%	5.50%
Total fixed rate debt		$ 355,328,000	$ 355,328,000	355,328,000
Committed amount		$ 355,300,000	$ 355,300,000
Joint venture | 650 Fifth Avenue
Debt Instrument [Line Items]
Economic Interest (as a percent)		50.00%	50.00%
Joint venture | 650 Fifth Avenue | Mortgage loan
Debt Instrument [Line Items]
Interest rate, fixed rate debt (as a percent)		4.46%	4.46%
Total fixed rate debt		$ 210,000,000	$ 210,000,000	210,000,000
Long-term debt, carrying value		$ 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%	5.45%
Total fixed rate debt		$ 65,000,000	$ 65,000,000	65,000,000
Joint venture | 21 East 66th Street
Debt Instrument [Line Items]
Economic Interest (as a percent)		32.28%	32.28%
Interest rate, fixed rate debt (as a percent)		3.60%	3.60%
Interest rate, floating rate debt (as a percent)		2.75%	2.75%
Total fixed rate debt		$ 12,000,000	$ 12,000,000	12,000,000
Total floating rate debt		$ 1,628,000	$ 1,628,000	1,648,000
Joint venture | 919 Third Avenue
Debt Instrument [Line Items]
Economic Interest (as a percent)		51.00%	51.00%
Interest rate, fixed rate debt (as a percent)		5.12%	5.12%
Total fixed rate debt		$ 500,000,000	$ 500,000,000	0
Joint venture | 3 Columbus Circle
Debt Instrument [Line Items]
Economic Interest (as a percent)		48.90%	48.90%
Interest rate, fixed rate debt (as a percent)		3.61%	3.61%
Total fixed rate debt		$ 350,000,000	$ 350,000,000	350,000,000
Joint venture | 1515 Broadway
Debt Instrument [Line Items]
Interest rate, fixed rate debt (as a percent)		3.93%	3.93%
Total fixed rate debt		$ 868,309,000	$ 868,309,000	872,528,000
Joint venture | 11 Madison Avenue
Debt Instrument [Line Items]
Economic Interest (as a percent)		60.00%	60.00%
Interest rate, fixed rate debt (as a percent)		3.84%	3.84%
Total fixed rate debt		$ 1,400,000,000	$ 1,400,000,000	1,400,000,000
Joint venture | 800 Third Avenue
Debt Instrument [Line Items]
Economic Interest (as a percent)		60.52%	60.52%
Interest rate, fixed rate debt (as a percent)		3.37%	3.37%
Total fixed rate debt		$ 177,000,000	$ 177,000,000	177,000,000
Joint venture | East 400 Street 57
Debt Instrument [Line Items]
Economic Interest (as a percent)		41.00%	41.00%
Interest rate, fixed rate debt (as a percent)		3.00%	3.00%
Total fixed rate debt		$ 100,000,000	$ 100,000,000	100,000,000
Joint venture | Worldwide Plaza
Debt Instrument [Line Items]
Interest rate, fixed rate debt (as a percent)		3.98%	3.98%
Total fixed rate debt		$ 1,200,000,000	$ 1,200,000,000	1,200,000,000
Joint venture | Stonehenge Portfolio
Debt Instrument [Line Items]
Interest rate, fixed rate debt (as a percent)		4.25%	4.25%
Interest rate, floating rate debt (as a percent)		1.40%	1.40%
Total fixed rate debt		$ 306,990,000	$ 306,990,000	357,282,000
Total floating rate debt		$ 38,000,000	$ 38,000,000	55,340,000
Joint venture | 1745 Broadway
Debt Instrument [Line Items]
Economic Interest (as a percent)		56.87%	56.87%
Interest rate, floating rate debt (as a percent)		1.85%	1.85%
Total floating rate debt		$ 345,000,000	$ 345,000,000	345,000,000
Joint venture | 1745 Broadway | Mortgage loan
Debt Instrument [Line Items]
Committed amount		375,000,000	375,000,000
Unfunded amount		$ 30,000,000	$ 30,000,000
Joint venture | 55 West 46th Street
Debt Instrument [Line Items]
Economic Interest (as a percent)		25.00%	25.00%
Interest rate, floating rate debt (as a percent)		2.13%	2.13%
Total floating rate debt		$ 172,511,000	$ 172,511,000	171,444,000
Committed amount		195,000,000	195,000,000
Unused borrowing capacity, amount		$ 22,500,000	$ 22,500,000
Joint venture | 175-225 Third Street Brooklyn, New York
Debt Instrument [Line Items]
Economic Interest (as a percent)		95.00%	95.00%
Interest rate, floating rate debt (as a percent)		1.00%	1.00%
Total floating rate debt		$ 40,000,000	$ 40,000,000	40,000,000
Joint venture | Jericho Plaza
Debt Instrument [Line Items]
Economic Interest (as a percent)		11.67%	11.67%
Interest rate, floating rate debt (as a percent)		4.15%	4.15%
Total floating rate debt		$ 81,922,000	$ 81,922,000	81,099,000
Extension option exercised,term		12 months
Term (in Years)			2 years
Face amount of loan		$ 100,000,000	$ 100,000,000
Joint venture | 724 Fifth Avenue
Debt Instrument [Line Items]
Economic Interest (as a percent)		50.00%	50.00%
Interest rate, floating rate debt (as a percent)		2.42%	2.42%
Total floating rate debt		$ 275,000,000	$ 275,000,000	275,000,000
Joint venture | 1552 Broadway
Debt Instrument [Line Items]
Economic Interest (as a percent)		50.00%	50.00%
Interest rate, floating rate debt (as a percent)		2.65%	2.65%
Total floating rate debt		$ 195,000,000	$ 195,000,000	195,000,000
Joint venture | 280 Park Avenue
Debt Instrument [Line Items]
Economic Interest (as a percent)		50.00%	50.00%
Interest rate, floating rate debt (as a percent)		1.73%	1.73%
Total floating rate debt		$ 1,200,000,000	$ 1,200,000,000	1,200,000,000
Joint venture | 121 Greene Street
Debt Instrument [Line Items]
Economic Interest (as a percent)		50.00%	50.00%
Interest rate, floating rate debt (as a percent)		1.50%	1.50%
Total floating rate debt		$ 15,000,000	$ 15,000,000	15,000,000
Joint venture | 10 East 53rd Street
Debt Instrument [Line Items]
Economic Interest (as a percent)		55.00%	55.00%
Interest rate, floating rate debt (as a percent)		2.25%	2.25%
Total floating rate debt		$ 170,000,000	$ 170,000,000	170,000,000
Joint venture | 131-137 Spring Street
Debt Instrument [Line Items]
Economic Interest (as a percent)		20.00%	20.00%
Interest rate, floating rate debt (as a percent)		1.55%	1.55%
Total floating rate debt		$ 141,000,000	$ 141,000,000	141,000,000
Joint venture | 11 West 34th Street
Debt Instrument [Line Items]
Economic Interest (as a percent)		30.00%	30.00%
Interest rate, floating rate debt (as a percent)		1.45%	1.45%
Total floating rate debt		$ 23,000,000	$ 23,000,000	23,000,000
Joint venture | 100 Park Avenue
Debt Instrument [Line Items]
Economic Interest (as a percent)		49.90%	49.90%
Interest rate, floating rate debt (as a percent)		1.75%	1.75%
Total floating rate debt		$ 360,000,000	$ 360,000,000	360,000,000
Joint venture | One Vanderbilt
Debt Instrument [Line Items]
Economic Interest (as a percent)		71.01%	71.01%
Interest rate, floating rate debt (as a percent)		3.50%	3.50%
Total floating rate debt		$ 375,000,000	$ 375,000,000	355,535,000
Joint venture | One Vanderbilt | Construction Loans
Debt Instrument [Line Items]
Term (in Years)			5 years
Maximum facility capacity		$ 1,500,000,000.0	$ 1,500,000,000.0
Credit facility, interest rate (as a percent)			3.50%
Number of extension options | extension			2
Period of extension options			1 year
Joint venture | 605 West 42nd Street
Debt Instrument [Line Items]
Economic Interest (as a percent)		20.00%	20.00%
Interest rate, floating rate debt (as a percent)		1.44%	1.44%
Total floating rate debt		$ 550,000,000	$ 550,000,000	$ 550,000,000
Joint venture | Subsequent Event | 724 Fifth Avenue
Debt Instrument [Line Items]
Extension option exercised,term	12 months
Mezzanine Loan, Initial Maturity October 2022 [Member] | Joint venture | 650 Fifth Avenue | Mezzanine loans
Debt Instrument [Line Items]
Long-term debt, carrying value		65,000,000	65,000,000
Secured Debt, Initial Maturity November 2017 [Member] | Joint venture | Stonehenge Portfolio
Debt Instrument [Line Items]
Total fixed rate debt		136,400,000	136,400,000
Secured Debt, Initial Maturity February 2027 [Member] | Joint venture | Stonehenge Portfolio
Debt Instrument [Line Items]
Total fixed rate debt		$ 170,600,000	$ 170,600,000