Investments in Unconsolidated Joint Ventures (Mortgages and Other Loans Payable)(Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Feb. 28, 2017	Jan. 31, 2017	Oct. 31, 2016	Dec. 31, 2015
Debt Instrument [Line Items]
Total fixed rate debt	$ 3,518,127				$ 5,816,010
Total floating rate debt	622,585				1,205,494
Total fixed rate and floating rate debt	4,140,712				7,021,504
Deferred financing costs, net	$ (82,300)				(130,300)
1745 Broadway
Debt Instrument [Line Items]
Ownership Interest	56.88%
521 Fifth Avenue
Debt Instrument [Line Items]
Ownership Interest	50.50%
717 Fifth Avenue
Debt Instrument [Line Items]
Ownership Interest	10.92%
21 East 66th Street
Debt Instrument [Line Items]
Ownership Interest	32.28%
3 Columbus Circle
Debt Instrument [Line Items]
Ownership Interest	48.90%
11 Madison Avenue
Debt Instrument [Line Items]
Total fixed rate debt	$ 0				1,400,000
Ownership Interest	60.00%
800 Third Avenue
Debt Instrument [Line Items]
Ownership Interest	60.52%
400 East 57th Street
Debt Instrument [Line Items]
Total fixed rate debt	$ 0				67,644
Ownership Interest	51.00%
Stonehenge Portfolio
Debt Instrument [Line Items]
Ownership Interest				90.00%
280 Park Avenue
Debt Instrument [Line Items]
Ownership Interest	50.00%
10 East 53rd Street
Debt Instrument [Line Items]
Ownership Interest	55.00%
724 Fifth Avenue
Debt Instrument [Line Items]
Ownership Interest	50.00%
55 West 46th Street
Debt Instrument [Line Items]
Ownership Interest	25.00%
175-225 Third Street Brooklyn, New York
Debt Instrument [Line Items]
Ownership Interest	95.00%
605 West 42nd Street
Debt Instrument [Line Items]
Ownership Interest	20.00%
650 Fifth Avenue
Debt Instrument [Line Items]
Ownership Interest	50.00%
121 Greene Street
Debt Instrument [Line Items]
Ownership Interest	50.00%
131-137 Spring Street
Debt Instrument [Line Items]
Ownership Interest	20.00%
11 West 34th Street
Debt Instrument [Line Items]
Ownership Interest	30.00%
100 Park Avenue
Debt Instrument [Line Items]
Ownership Interest	49.90%
Joint venture
Debt Instrument [Line Items]
Total fixed rate debt	$ 3,566,846				2,645,131
Total floating rate debt	2,982,528				1,663,982
Total fixed rate and floating rate debt	6,549,374				4,309,113
Deferred financing costs, net	(95,408)				(42,565)
Total joint venture mortgages and other loans payable, net	6,453,966				4,266,548
Joint venture | 1745 Broadway
Debt Instrument [Line Items]
Total fixed rate debt	340,000				340,000
Joint venture | 521 Fifth Avenue
Debt Instrument [Line Items]
Total fixed rate debt	170,000				170,000
Joint venture | 717 Fifth Avenue | Mortgage loan
Debt Instrument [Line Items]
Committed amount	300,000
Joint venture | 717 Fifth Avenue | Mezzanine loans
Debt Instrument [Line Items]
Committed amount	355,300
Joint venture | 717 Fifth Avenue | Secured Debt, Initial Maturity July 2022 [Member]
Debt Instrument [Line Items]
Total fixed rate debt	300,000				300,000
Joint venture | 717 Fifth Avenue | Secured Debt, Initial Maturity July 2022, 2 [Member]
Debt Instrument [Line Items]
Total fixed rate debt	355,328				325,704
Joint venture | 21 East 66th Street
Debt Instrument [Line Items]
Total fixed rate debt	12,000				12,000
Total floating rate debt	1,726				1,805
Joint venture | 3 Columbus Circle
Debt Instrument [Line Items]
Total fixed rate debt	350,000				350,000
Joint venture | 11 Madison Avenue
Debt Instrument [Line Items]
Total fixed rate debt	1,400,000				0
Joint venture | 800 Third Avenue
Debt Instrument [Line Items]
Total fixed rate debt	177,000				20,910
Joint venture | 400 East 57th Street
Debt Instrument [Line Items]
Total fixed rate debt	100,000				0
Joint venture | Stonehenge Portfolio
Debt Instrument [Line Items]
Total fixed rate debt	362,518				430,627
Total floating rate debt	65,577				10,500
Ownership Interest				10.00%
Joint venture | Stonehenge Portfolio | Initial Maturity April 2017
Debt Instrument [Line Items]
Total fixed rate debt	13,100
Joint venture | Stonehenge Portfolio | Initial Maturity November 2017
Debt Instrument [Line Items]
Total fixed rate debt	34,500
Joint venture | Stonehenge Portfolio | Initial Maturity August 2019
Debt Instrument [Line Items]
Total fixed rate debt	139,700
Joint venture | Stonehenge Portfolio | Initial Maturity June 2024
Debt Instrument [Line Items]
Total fixed rate debt	175,200
Joint venture | Stonehenge Portfolio | Floating-Rate Mortgage Maturing in June 2017
Debt Instrument [Line Items]
Total floating rate debt	55,300
Joint venture | Stonehenge Portfolio | Floating-Rate Mortgage Maturing in December 2017
Debt Instrument [Line Items]
Total floating rate debt	10,200
Joint venture | 280 Park Avenue
Debt Instrument [Line Items]
Total fixed rate debt	0				692,963
Total floating rate debt	900,000				30,000
Joint venture | 7 Renaissance
Debt Instrument [Line Items]
Total fixed rate debt	0				2,927
Joint venture | 10 East 53rd Street
Debt Instrument [Line Items]
Total floating rate debt	125,000				125,000
Joint venture | 724 Fifth Avenue
Debt Instrument [Line Items]
Total floating rate debt	275,000				275,000
Joint venture | 1552 Broadway
Debt Instrument [Line Items]
Total floating rate debt	185,410				190,409
Joint venture | 1552 Broadway | Mortgage loan
Debt Instrument [Line Items]
Committed amount	145,000
Unfunded amount	600
Joint venture | 1552 Broadway | Mezzanine loans
Debt Instrument [Line Items]
Committed amount	41,500
Unfunded amount	500
Joint venture | 55 West 46th Street
Debt Instrument [Line Items]
Total floating rate debt	157,322				150,000
Joint venture | 55 West 46th Street | Mortgage loan
Debt Instrument [Line Items]
Committed amount	190,000
Unfunded amount	32,700
Joint venture | 175-225 Third Street Brooklyn, New York
Debt Instrument [Line Items]
Total floating rate debt	40,000				40,000
Joint venture | Jericho Plaza
Debt Instrument [Line Items]
Total floating rate debt	$ 76,993				163,750
Ownership Interest	11.67%
Term (in Years)	2 years
Face amount of loan	$ 100,000
Joint venture | 605 West 42nd Street
Debt Instrument [Line Items]
Total floating rate debt	539,000				0
Joint venture | 650 Fifth Avenue
Debt Instrument [Line Items]
Total floating rate debt	77,500				65,000
Joint venture | 650 Fifth Avenue | Mortgage loan
Debt Instrument [Line Items]
Committed amount	97,000
Unfunded amount	19,500
Joint venture | 121 Greene Street
Debt Instrument [Line Items]
Total floating rate debt	15,000				15,000
Joint venture | 131-137 Spring Street
Debt Instrument [Line Items]
Total floating rate debt	141,000				141,000
Joint venture | 11 West 34th Street
Debt Instrument [Line Items]
Total floating rate debt	23,000				23,000
Joint venture | 100 Park Avenue
Debt Instrument [Line Items]
Total floating rate debt	360,000				360,000
Joint venture | 33 Beekman
Debt Instrument [Line Items]
Total floating rate debt	$ 0				$ 73,518
Weighted Average | Joint venture | 1745 Broadway
Debt Instrument [Line Items]
Interest rate, fixed rate debt (as a percent)	5.68%
Weighted Average | Joint venture | 521 Fifth Avenue
Debt Instrument [Line Items]
Interest rate, fixed rate debt (as a percent)	3.73%
Weighted Average | Joint venture | 717 Fifth Avenue | Secured Debt, Initial Maturity July 2022 [Member]
Debt Instrument [Line Items]
Interest rate, fixed rate debt (as a percent)	4.45%
Weighted Average | Joint venture | 717 Fifth Avenue | Secured Debt, Initial Maturity July 2022, 2 [Member]
Debt Instrument [Line Items]
Interest rate, fixed rate debt (as a percent)	6.52%
Weighted Average | Joint venture | 21 East 66th Street
Debt Instrument [Line Items]
Interest rate, fixed rate debt (as a percent)	3.60%
Interest rate, floating rate debt (as a percent)	3.00%
Weighted Average | Joint venture | 3 Columbus Circle
Debt Instrument [Line Items]
Interest rate, fixed rate debt (as a percent)	3.61%
Weighted Average | Joint venture | 11 Madison Avenue
Debt Instrument [Line Items]
Interest rate, fixed rate debt (as a percent)	3.84%
Weighted Average | Joint venture | 800 Third Avenue
Debt Instrument [Line Items]
Interest rate, fixed rate debt (as a percent)	3.24%
Weighted Average | Joint venture | 400 East 57th Street
Debt Instrument [Line Items]
Interest rate, fixed rate debt (as a percent)	3.00%
Weighted Average | Joint venture | Stonehenge Portfolio
Debt Instrument [Line Items]
Interest rate, fixed rate debt (as a percent)	4.19%
Interest rate, floating rate debt (as a percent)	5.71%
Weighted Average | Joint venture | 280 Park Avenue
Debt Instrument [Line Items]
Interest rate, floating rate debt (as a percent)	2.51%
Weighted Average | Joint venture | 10 East 53rd Street
Debt Instrument [Line Items]
Interest rate, floating rate debt (as a percent)	2.98%
Weighted Average | Joint venture | 724 Fifth Avenue
Debt Instrument [Line Items]
Interest rate, floating rate debt (as a percent)	2.90%
Weighted Average | Joint venture | 1552 Broadway
Debt Instrument [Line Items]
Interest rate, floating rate debt (as a percent)	4.68%
Weighted Average | Joint venture | 55 West 46th Street
Debt Instrument [Line Items]
Interest rate, floating rate debt (as a percent)	2.78%
Weighted Average | Joint venture | 175-225 Third Street Brooklyn, New York
Debt Instrument [Line Items]
Interest rate, floating rate debt (as a percent)	4.51%
Weighted Average | Joint venture | Jericho Plaza
Debt Instrument [Line Items]
Interest rate, floating rate debt (as a percent)	4.64%
Weighted Average | Joint venture | 605 West 42nd Street
Debt Instrument [Line Items]
Interest rate, floating rate debt (as a percent)	2.91%
Weighted Average | Joint venture | 650 Fifth Avenue
Debt Instrument [Line Items]
Interest rate, floating rate debt (as a percent)	4.10%
Weighted Average | Joint venture | 121 Greene Street
Debt Instrument [Line Items]
Interest rate, floating rate debt (as a percent)	1.98%
Weighted Average | Joint venture | 131-137 Spring Street
Debt Instrument [Line Items]
Interest rate, floating rate debt (as a percent)	2.03%
Weighted Average | Joint venture | 11 West 34th Street
Debt Instrument [Line Items]
Interest rate, floating rate debt (as a percent)	1.93%
Weighted Average | Joint venture | 100 Park Avenue
Debt Instrument [Line Items]
Interest rate, floating rate debt (as a percent)	2.23%
Subsequent Event | Joint venture | 1745 Broadway
Debt Instrument [Line Items]
Interest rate, fixed rate debt (as a percent)			1.85%
Total fixed rate debt			$ 375,000
Subsequent Event | Joint venture | 10 East 53rd Street
Debt Instrument [Line Items]
Total floating rate debt		$ 170,000
LIBOR | Subsequent Event | Joint venture | 10 East 53rd Street
Debt Instrument [Line Items]
Interest rate, floating rate debt (as a percent)		2.25%