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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
	Mar. 31, 2016	Feb. 29, 2016	Mar. 31, 2016	Mar. 31, 2015	Dec. 31, 2015
Schedule of Equity Method Investments [Line Items]
Acquisition, development and construction arrangements, carrying value	$ 145,774		$ 145,774		$ 145,878
Gain (Loss) on Sale			$ 0	$ 12,983
Employee compensation award	$ 500
1 Jericho Plaza
Schedule of Equity Method Investments [Line Items]
Ownership Percentage		66.11%
Gross Asset Valuation		$ 95,200
Gain (Loss) on Sale		$ 3,300
7 Renaissance Square
Schedule of Equity Method Investments [Line Items]
Ownership Percentage	50.00%		50.00%
Gross Asset Valuation	$ 20,700		$ 20,700
Gain (Loss) on Sale	$ 4,200
EOP Denver
Schedule of Equity Method Investments [Line Items]
Ownership Percentage	4.79%		4.79%
Gross Asset Valuation	$ 180,700		$ 180,700
Gain (Loss) on Sale	2,800
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	$ 45,774		$ 45,774		$ 45,942