Investments in Unconsolidated Joint Ventures (Details) $ in Thousands	1 Months Ended					3 Months Ended		9 Months Ended
	Oct. 31, 2015 USD ($)	Aug. 31, 2015 USD ($) ft²	Jul. 31, 2015 USD ($)	Mar. 31, 2015 USD ($)	Feb. 28, 2015 USD ($) property	Sep. 30, 2015 USD ($) ft² unit	Sep. 30, 2014 USD ($)	Sep. 30, 2015 USD ($) ft² unit	Sep. 30, 2014 USD ($)	Dec. 31, 2014 USD ($)
General information on each joint venture
Net equity investment in VIEs in which the entity is not primary beneficiary						$ 100,200		$ 100,200		$ 146,200
Approximate Square Feet (sqft) | ft²						35,843,296		35,843,296
Loans Receivable, Gross, Commercial, Acquisition						$ 145,763		$ 145,763		145,875
Gain (Loss) on Sale						$ 0	$ 29,507	$ 12,983	$ 144,242
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
Additional Ownership Percentage Acquired (less than for the 1.0%)				17.56%
Additional Cost for additional interest in joint venture				$ 67,500
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)						20.26%		20.26%
Economic Interest (as a percent)						20.26%		20.26%
Approximate Square Feet (sqft) | ft²						640,000		640,000
Acquisition Price						$ 210,000		$ 210,000
600 Lexington Avenue
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						303,515		303,515
Acquisition Price						$ 193,000		$ 193,000
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
7 Renaissance
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						65,641		65,641
Acquisition Price						$ 4,000		$ 4,000
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
33 Beekman
General information on each joint venture
Ownership Interest (as a percent)						45.90%		45.90%
Economic Interest (as a percent)						45.90%		45.90%
Approximate Square Feet (sqft) | ft²						163,500		163,500
Acquisition Price						$ 31,000		$ 31,000
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
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
Acquisition Price						$ 0		$ 0
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					$ 40,200	$ 36,668		$ 36,668
Additional Ownership Percentage Acquired (less than for the 1.0%)			1.00%
Additional Cost for additional interest in joint venture			$ 1,100
Increase in real estate investment ownership interest					$ 3,500
Number of Joint Venture Properties with an Increased Ownership Interest | property					6
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
1552 Broadway
General information on each joint venture
Approximate Square Feet (sqft) | ft²						13,045		13,045
Three Retail and Two Residential Units | 21 East 66th Street
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
Three Residential Units | 21 East 66th Street
General information on each joint venture
Ownership Interest (as a percent)						16.14%		16.14%
Number Of Residential Units | unit						3		3
Series M Preferred Units
General information on each joint venture
Preferred Units (as a percent)					3.75%
Series M Preferred Units | Stonehenge Portfolio
General information on each joint venture
Issuance of aggregate liquidation preference of preferred units					$ 40,000
Mezzanine loan and preferred equity due March 2016
General information on each joint venture
Loans Receivable, Gross, Commercial, Acquisition						$ 99,856		$ 99,856		99,629
Mezzanine loan due February 2022
General information on each joint venture
Loans Receivable, Gross, Commercial, Acquisition						$ 45,907		$ 45,907		$ 46,246
Subsequent Event | 33 Beekman
General information on each joint venture
Agreed sale consideration	$ 196,000
131-137 Spring Street
General information on each joint venture
Approximate Square Feet (sqft) | ft²		68,342
Agreed sale consideration		$ 277,800
Gain (Loss) on Sale		$ 101,100
Ownership percentage in disposed asset		80.00%
The Meadows
General information on each joint venture
Ownership Interest (as a percent)						50.00%		50.00%
Gross Asset Valuation						$ 121,100		$ 121,100
Gain (Loss) on Sale								$ (1,600)
315 West 36th Street
General information on each joint venture
Ownership Interest (as a percent)						35.50%		35.50%
Gross Asset Valuation						$ 115,000		$ 115,000
Gain (Loss) on Sale								$ 16,300