Investments in Unconsolidated Joint Ventures (Mortgages and Other Loans Payable)(Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2016	Dec. 31, 2015
Debt Instrument [Line Items]
Total fixed rate debt	$ 3,563,685	$ 5,816,010
Total floating rate debt	527,990	1,205,494
Total fixed rate and floating rate debt	4,091,675	7,021,504
Deferred financing costs, net	$ (101,500)	(130,300)
1745 Broadway
Debt Instrument [Line Items]
Ownership Percentage	56.88%
521 Fifth Avenue
Debt Instrument [Line Items]
Ownership Percentage	50.50%
717 Fifth Avenue
Debt Instrument [Line Items]
Ownership Percentage	10.92%
21 East 66th Street
Debt Instrument [Line Items]
Ownership Percentage	32.28%
3 Columbus Circle
Debt Instrument [Line Items]
Ownership Percentage	48.90%
11 Madison Avenue
Debt Instrument [Line Items]
Total fixed rate debt	$ 0	1,400,000
Ownership Percentage	60.00%
800 Third Avenue
Debt Instrument [Line Items]
Ownership Percentage	60.52%
280 Park Avenue
Debt Instrument [Line Items]
Ownership Percentage	50.00%
175-225 Third Street Brooklyn, New York
Debt Instrument [Line Items]
Ownership Percentage	95.00%
10 East 53rd Street
Debt Instrument [Line Items]
Ownership Percentage	55.00%
724 Fifth Avenue
Debt Instrument [Line Items]
Ownership Percentage	50.00%
55 West 46th Street
Debt Instrument [Line Items]
Ownership Percentage	25.00%
605 West 42nd Street
Debt Instrument [Line Items]
Ownership Percentage	20.00%
650 Fifth Avenue
Debt Instrument [Line Items]
Ownership Percentage	50.00%
121 Greene Street
Debt Instrument [Line Items]
Ownership Percentage	50.00%
131-137 Spring Street
Debt Instrument [Line Items]
Ownership Percentage	20.00%
11 West 34th Street
Debt Instrument [Line Items]
Ownership Percentage	30.00%
100 Park Avenue
Debt Instrument [Line Items]
Ownership Percentage	49.90%
Joint venture
Debt Instrument [Line Items]
Total fixed rate debt	$ 3,468,583	2,645,131
Total floating rate debt	2,964,349	1,663,982
Total fixed rate and floating rate debt	6,432,932	4,309,113
Deferred financing costs, net	(100,426)	(42,565)
Total joint venture mortgages and other loans payable, net	6,332,506	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 | Initial Maturity July 2022
Debt Instrument [Line Items]
Total fixed rate debt	300,000	300,000
Joint venture | 717 Fifth Avenue | Initial Maturity July 2022, 2
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	1,746	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 | Stonehenge Portfolio
Debt Instrument [Line Items]
Total fixed rate debt	364,255	430,627
Total floating rate debt	65,664	10,500
Joint venture | Stonehenge Portfolio | Initial Maturity April 2017
Debt Instrument [Line Items]
Total fixed rate debt	13,200
Joint venture | Stonehenge Portfolio | Initial Maturity November 2017
Debt Instrument [Line Items]
Total fixed rate debt	34,600
Joint venture | Stonehenge Portfolio | Initial Maturity August 2019
Debt Instrument [Line Items]
Total fixed rate debt	140,300
Joint venture | Stonehenge Portfolio | Initial Maturity June 2024
Debt Instrument [Line Items]
Total fixed rate debt	176,100
Joint venture | Stonehenge Portfolio | Initial Maturity June 2017
Debt Instrument [Line Items]
Total floating rate debt	55,300
Joint venture | Stonehenge Portfolio | Initial Maturity December 2017
Debt Instrument [Line Items]
Total floating rate debt	10,300
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 Square
Debt Instrument [Line Items]
Total fixed rate debt	0	2,927
Joint venture | 175-225 Third Street Brooklyn, New York
Debt Instrument [Line Items]
Total floating rate debt	40,000	40,000
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	151,536	150,000
Joint venture | 55 West 46th Street | Mortgage loan
Debt Instrument [Line Items]
Committed amount	190,000
Unfunded amount	38,500
Joint venture | Jericho Plaza
Debt Instrument [Line Items]
Total floating rate debt	$ 76,993	163,750
Ownership Percentage	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	65,000	65,000
Joint venture | 650 Fifth Avenue | Mortgage loan
Debt Instrument [Line Items]
Committed amount	97,000
Unfunded amount	32,000
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 | Initial Maturity July 2022
Debt Instrument [Line Items]
Interest rate, fixed rate debt (as a percent)	4.45%
Weighted Average | Joint venture | 717 Fifth Avenue | Initial Maturity July 2022, 2
Debt Instrument [Line Items]
Interest rate, fixed rate debt (as a percent)	5.50%
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)	3.37%
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)	5.81%
Weighted Average | Joint venture | 280 Park Avenue
Debt Instrument [Line Items]
Interest rate, floating rate debt (as a percent)	2.49%
Weighted Average | Joint venture | 175-225 Third Street Brooklyn, New York
Debt Instrument [Line Items]
Interest rate, floating rate debt (as a percent)	4.50%
Weighted Average | Joint venture | 10 East 53rd Street
Debt Instrument [Line Items]
Interest rate, floating rate debt (as a percent)	2.99%
Weighted Average | Joint venture | 724 Fifth Avenue
Debt Instrument [Line Items]
Interest rate, floating rate debt (as a percent)	2.91%
Weighted Average | Joint venture | 1552 Broadway
Debt Instrument [Line Items]
Interest rate, floating rate debt (as a percent)	4.71%
Weighted Average | Joint venture | 55 West 46th Street
Debt Instrument [Line Items]
Interest rate, floating rate debt (as a percent)	2.79%
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)	2.85%
Weighted Average | Joint venture | 650 Fifth Avenue
Debt Instrument [Line Items]
Interest rate, floating rate debt (as a percent)	4.17%
Weighted Average | Joint venture | 121 Greene Street
Debt Instrument [Line Items]
Interest rate, floating rate debt (as a percent)	1.99%
Weighted Average | Joint venture | 131-137 Spring Street
Debt Instrument [Line Items]
Interest rate, floating rate debt (as a percent)	2.04%
Weighted Average | Joint venture | 11 West 34th Street
Debt Instrument [Line Items]
Interest rate, floating rate debt (as a percent)	1.94%
Weighted Average | Joint venture | 100 Park Avenue
Debt Instrument [Line Items]
Interest rate, floating rate debt (as a percent)	2.24%