Investments in Unconsolidated Joint Ventures (Acquisition, Development and Construction Arrangements/Sale of Joint Venture Interest or Property)(Details) - USD ($) $ in Thousands	1 Months Ended			3 Months Ended		6 Months Ended
	May 31, 2016	Mar. 31, 2016	Feb. 29, 2016	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015	Dec. 31, 2015
Schedule of Equity Method Investments [Line Items]
Acquisition, development and construction arrangements, carrying value				$ 145,719		$ 145,719		$ 145,878
Gain on Sale				0	$ 0	0	$ 12,983
Employee compensation award						1,100
1 Jericho Plaza
Schedule of Equity Method Investments [Line Items]
Ownership Percentage		11.67%	66.11%					77.78%
Gross Asset Valuation			$ 95,200
Gain on Sale			$ 3,300
7 Renaissance Square
Schedule of Equity Method Investments [Line Items]
Ownership Percentage		50.00%
Gross Asset Valuation		$ 20,700
Gain on Sale		$ 4,200
EOP Denver
Schedule of Equity Method Investments [Line Items]
Ownership Percentage		4.79%
Gross Asset Valuation		$ 180,700
Gain on Sale		$ 2,800
33 Beekman
Schedule of Equity Method Investments [Line Items]
Ownership Percentage	45.90%
Gross Asset Valuation	$ 196,000
Gain on Sale	33,000
Promote recognized	$ 10,800
Mezzanine loan and preferred equity due March 2017
Schedule of Equity Method Investments [Line Items]
Acquisition, development and construction arrangements, carrying value				100,000		100,000		$ 99,936
Mezzanine loan due February 2022
Schedule of Equity Method Investments [Line Items]
Acquisition, development and construction arrangements, carrying value				$ 45,719		$ 45,719		$ 45,942