Investments in Unconsolidated Joint Ventures Investments in Unconsolidated Joint Ventures (Acquisition, Development and Construction Arrangements/Sale of Joint Venture Interest or Property)(Details) $ in Thousands	1 Months Ended							12 Months Ended
	Sep. 30, 2015 USD ($)	Aug. 31, 2015 USD ($)	Sep. 30, 2014 USD ($)	May. 31, 2014 USD ($) unit	Mar. 31, 2014 USD ($)	Jan. 31, 2014 USD ($) ft²	Dec. 31, 2013 USD ($) joint_venture	Dec. 31, 2015 USD ($) ft²	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($) joint_venture property
Schedule of Equity Method Investments [Line Items]
Acquisition, development and construction arrangements, carrying value								$ 145,878	$ 145,875
Gain (Loss) on Sale								$ 14,122	163,059	$ 14,900
Approximate Square Feet (sqft) | ft²								35,581,060
West Coast Office portfolio
Schedule of Equity Method Investments [Line Items]
Number of apartment units | unit				1
West Coast Office Portfolio
Schedule of Equity Method Investments [Line Items]
Number of Joint Venture Properties with an Increased Ownership Interest | joint_venture							2			2
Mezzanine loan and preferred equity due March 2016
Schedule of Equity Method Investments [Line Items]
Acquisition, development and construction arrangements, carrying value								$ 99,936	99,629
Mezzanine loan due February 2022
Schedule of Equity Method Investments [Line Items]
Acquisition, development and construction arrangements, carrying value								$ 45,942	$ 46,246
The Meadows
Schedule of Equity Method Investments [Line Items]
Ownership Percentage		50.00%
Gross Asset Valuation		$ 121,100
Gain (Loss) on Sale		$ (1,600)
315 West 36th Street
Schedule of Equity Method Investments [Line Items]
Ownership Percentage	35.50%
Gross Asset Valuation	$ 115,000
Gain (Loss) on Sale	16,300
Employee compensation award	$ 1,200
180 Broadway
Schedule of Equity Method Investments [Line Items]
Ownership Percentage			25.50%
Gross Asset Valuation			$ 222,500
Gain (Loss) on Sale			16,500
Promote recognized			$ 3,300
747 Madison Avenue
Schedule of Equity Method Investments [Line Items]
Ownership Percentage				33.33%
Gross Asset Valuation				$ 160,000
Gain (Loss) on Sale				0
Promote recognized				10,300
Preferred equity investment				30,000
Deferred gain on sale				$ 13,100
West Coast Office portfolio
Schedule of Equity Method Investments [Line Items]
Ownership Percentage					42.02%
Gross Asset Valuation					$ 756,000
Gain (Loss) on Sale					$ 85,600
Number of apartment units | unit				5
21-25 West 34th Street
Schedule of Equity Method Investments [Line Items]
Ownership Percentage						49.90%
Gross Asset Valuation						$ 114,900
Gain (Loss) on Sale						$ 20,900
Approximate Square Feet (sqft) | ft²						91,300
27-29 West 34th Street
Schedule of Equity Method Investments [Line Items]
Ownership Percentage							50.00%			50.00%
Gross Asset Valuation							$ 70,100			$ 70,100
Gain (Loss) on Sale							7,600
Employee compensation award							$ 1,500
West Coast Office Portfolio
Schedule of Equity Method Investments [Line Items]
Ownership Percentage							42.04%			42.04%
Gross Asset Valuation							$ 224,300			$ 224,300
Gain (Loss) on Sale										$ 2,100
Number of properties sold | property										3
Economic Interest (as a percent)							43.74%			43.74%
West Coast Office Portfolio, Two Joint Venture Partners
Schedule of Equity Method Investments [Line Items]
Ownership Percentage							14.39%			14.39%
Herald Center | Preferred equity
Schedule of Equity Method Investments [Line Items]
Interest rate (as a percent)							8.75%			8.75%
Joint venture | 747 Madison Avenue | 30 East 65th Street
Schedule of Equity Method Investments [Line Items]
Investment's ownership percentage in joint venture				100.00%