Investments in Unconsolidated Joint Ventures (Details) $ in Thousands	1 Months Ended	3 Months Ended	9 Months Ended
	Aug. 31, 2016 ft²	Sep. 30, 2016 USD ($) ft² unit	Sep. 30, 2016 USD ($) ft² unit note	Apr. 30, 2016	Dec. 31, 2015 USD ($)	Jul. 31, 2014 USD ($)
General information on each joint venture
Net equity investment in VIEs in which the entity is not primary beneficiary		$ 220,700	$ 220,700		$ 39,700
Approximate Square Feet (sqft) | ft²		32,918,879	32,918,879
Joint venture
General information on each joint venture
Equity method investment, assets		$ 10,478,811	$ 10,478,811		$ 6,984,186
Mezzanine Loan Repaid in April 2016
General information on each joint venture
Ownership Interest (as a percent)				20.00%
Mezzanine loan						$ 50,000
100 Park Avenue
General information on each joint venture
Ownership Interest (as a percent)		49.90%	49.90%
Economic Interest (as a percent)		49.90%	49.90%
Approximate Square Feet (sqft) | ft²		834,000	834,000
Acquisition Price		$ 95,800	$ 95,800
717 Fifth Avenue
General information on each joint venture
Ownership Interest (as a percent)		10.92%	10.92%
Economic Interest (as a percent)		10.92%	10.92%
Approximate Square Feet (sqft) | ft²		119,500	119,500
Acquisition Price		$ 251,900	$ 251,900
800 Third Avenue
General information on each joint venture
Ownership Interest (as a percent)		60.52%	60.52%
Economic Interest (as a percent)		60.52%	60.52%
Approximate Square Feet (sqft) | ft²		526,000	526,000
Acquisition Price		$ 285,000	$ 285,000
1745 Broadway
General information on each joint venture
Ownership Interest (as a percent)		56.88%	56.88%
Economic Interest (as a percent)		56.88%	56.88%
Approximate Square Feet (sqft) | ft²		674,000	674,000
Acquisition Price		$ 520,000	$ 520,000
Jericho Plaza
General information on each joint venture
Ownership Interest (as a percent)		11.67%	11.67%		77.78%
Economic Interest (as a percent)		11.67%	11.67%
Approximate Square Feet (sqft) | ft²		640,000	640,000
Acquisition Price		$ 210,000	$ 210,000
11 West 34th Street
General information on each joint venture
Ownership Interest (as a percent)		30.00%	30.00%
Economic Interest (as a percent)		30.00%	30.00%
Approximate Square Feet (sqft) | ft²		17,150	17,150
Acquisition Price		$ 10,800	$ 10,800
3 Columbus Circle
General information on each joint venture
Ownership Interest (as a percent)		48.90%	48.90%
Economic Interest (as a percent)		48.90%	48.90%
Approximate Square Feet (sqft) | ft²		741,500	741,500
Acquisition Price		$ 500,000	$ 500,000
280 Park Avenue
General information on each joint venture
Ownership Interest (as a percent)		50.00%	50.00%
Economic Interest (as a percent)		50.00%	50.00%
Approximate Square Feet (sqft) | ft²		1,219,158	1,219,158
Acquisition Price		$ 400,000	$ 400,000
1552-1560 Broadway
General information on each joint venture
Ownership Interest (as a percent)		50.00%	50.00%
Economic Interest (as a percent)		50.00%	50.00%
Approximate Square Feet (sqft) | ft²		35,897	35,897
Acquisition Price		$ 136,550	$ 136,550
724 Fifth Avenue
General information on each joint venture
Ownership Interest (as a percent)		50.00%	50.00%
Economic Interest (as a percent)		50.00%	50.00%
Approximate Square Feet (sqft) | ft²		65,040	65,040
Acquisition Price		$ 223,000	$ 223,000
10 East 53rd Street
General information on each joint venture
Ownership Interest (as a percent)		55.00%	55.00%
Economic Interest (as a percent)		55.00%	55.00%
Approximate Square Feet (sqft) | ft²		354,300	354,300
Acquisition Price		$ 252,500	$ 252,500
521 Fifth Avenue
General information on each joint venture
Ownership Interest (as a percent)		50.50%	50.50%
Economic Interest (as a percent)		50.50%	50.50%
Approximate Square Feet (sqft) | ft²		460,000	460,000
Acquisition Price		$ 315,000	$ 315,000
21 East 66th Street
General information on each joint venture
Ownership Interest (as a percent)		32.28%	32.28%
Economic Interest (as a percent)		32.28%	32.28%
Approximate Square Feet (sqft) | ft²		16,736	16,736
Acquisition Price		$ 75,000	$ 75,000
21 East 66th Street | Three Retail and Two Residential Units
General information on each joint venture
Ownership Interest (as a percent)		32.28%	32.28%
Number of Stores | unit		3	3
Number Of Residential Units | unit		2	2
21 East 66th Street | Three Residential Units
General information on each joint venture
Ownership Interest (as a percent)		16.14%	16.14%
Number Of Residential Units | unit		3	3
650 Fifth Avenue
General information on each joint venture
Ownership Interest (as a percent)		50.00%	50.00%
Economic Interest (as a percent)		50.00%	50.00%
Approximate Square Feet (sqft) | ft²		32,324	32,324
121 Greene Street
General information on each joint venture
Ownership Interest (as a percent)		50.00%	50.00%
Economic Interest (as a percent)		50.00%	50.00%
Approximate Square Feet (sqft) | ft²		7,131	7,131
Acquisition Price		$ 27,400	$ 27,400
175-225 Third Street Brooklyn, New York
General information on each joint venture
Ownership Interest (as a percent)		95.00%	95.00%
Economic Interest (as a percent)		95.00%	95.00%
Approximate Square Feet (sqft) | ft²		0	0
Acquisition Price		$ 74,600	$ 74,600
55 West 46th Street
General information on each joint venture
Ownership Interest (as a percent)		25.00%	25.00%
Economic Interest (as a percent)		25.00%	25.00%
Approximate Square Feet (sqft) | ft²		347,000	347,000
Acquisition Price		$ 295,000	$ 295,000
Stonehenge Portfolio
General information on each joint venture
Approximate Square Feet (sqft) | ft²		2,046,733	2,046,733
Acquisition Price		$ 36,668	$ 36,668
131-137 Spring Street
General information on each joint venture
Ownership Interest (as a percent)		20.00%	20.00%
Economic Interest (as a percent)		20.00%	20.00%
Approximate Square Feet (sqft) | ft²		68,342	68,342
Acquisition Price		$ 277,750	$ 277,750
76 11th Avenue
General information on each joint venture
Ownership Interest (as a percent)		33.33%	33.33%
Economic Interest (as a percent)		36.58%	36.58%
Approximate Square Feet (sqft) | ft²		764,000	764,000
Acquisition Price		$ 138,240	$ 138,240
Number of mezzanine notes secured by interests in the entity which owns 76 11th avenue | note			2
Exit fee, percentage owned			50.00%
605 West 42nd Street
General information on each joint venture
Ownership Interest (as a percent)		20.00%	20.00%
Economic Interest (as a percent)		20.00%	20.00%
Approximate Square Feet (sqft) | ft²		927,358	927,358
Acquisition Price		$ 759,000	$ 759,000
1552 Broadway
General information on each joint venture
Approximate Square Feet (sqft) | ft²		13,045	13,045
11 Madison Avenue
General information on each joint venture
Ownership Interest (as a percent)		60.00%	60.00%
Economic Interest (as a percent)		60.00%	60.00%
Approximate Square Feet (sqft) | ft²		2,314,000	2,314,000
Acquisition Price		$ 2,605,000	$ 2,605,000
Equity method investment, assets		2,700,000	2,700,000
Equity method investment, liabilities		1,500,000	1,500,000
Contingent consideration received at closing		$ 482,000	$ 482,000
333 East 22nd St
General information on each joint venture
Ownership Interest (as a percent)		33.33%	33.33%
Economic Interest (as a percent)		33.33%	33.33%
Approximate Square Feet (sqft) | ft²		26,926	26,926
Acquisition Price		$ 0	$ 0
Partners' Interest | 76 11th Avenue | Joint venture
General information on each joint venture
Exit fee, percentage owned			25.00%
Partners' Interest | 11 Madison Avenue | Joint venture
General information on each joint venture
Equity method investments, depreciation		$ 4,700	$ 4,700
11 Madison Avenue
General information on each joint venture
Approximate Square Feet (sqft) | ft²	2,314,000
Ownership percentage in disposed asset	40.00%