Real Estate Investments Dispositions (Details) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2015 USD ($) ft² property	Jun. 30, 2014 USD ($)	Jun. 30, 2015 USD ($) ft² property		Jun. 30, 2014 USD ($)
Significant Acquisitions and Disposals [Line Items]
Number of Properties | property		9		9
Sale Price		$ 147,500		$ 147,500
Gain/(Loss) On Sale		$ 1,571	$ 0	$ 10,590	[1]	$ 0
Rentable Square Feet | ft²		985,914		985,914
Provision for impairment on assets held for sale		$ 782	$ 0	$ 2,508		$ 0
Sold Property		$ 147,500		147,500
Original Disposition Target				180,000
Disposition Target				$ 300,000
100 Gateway Centre Parkway [Member]
Significant Acquisitions and Disposals [Line Items]
Number of Properties | property		1		1
Sale Price		$ 4,100		$ 4,100
Gain/(Loss) On Sale	[1],[2]			$ 0
Rentable Square Feet | ft²		74,991		74,991
Provision for impairment on assets held for sale				$ 800
Delaware Corporate Center I and II and Christiana Corporate Center [Member]
Significant Acquisitions and Disposals [Line Items]
Number of Properties | property		5		5
Sale Price		$ 50,100		$ 50,100
Gain/(Loss) On Sale	[1]			$ 1,600
Rentable Square Feet | ft²		485,182		485,182
Lake Merritt Tower [Member]
Significant Acquisitions and Disposals [Line Items]
Number of Properties | property		1		1
Sale Price		$ 65,000		$ 65,000
Gain/(Loss) On Sale	[1],[3]			$ 0
Rentable Square Feet | ft²		204,336		204,336
Provision for impairment on assets held for sale				$ 1,700
1000 Atrium Way and Libertyview [Member]
Significant Acquisitions and Disposals [Line Items]
Number of Properties | property		2		2
Sale Price		$ 28,300		$ 28,300
Gain/(Loss) On Sale	[1]			$ 9,000
Rentable Square Feet | ft²		221,405		221,405

[1] Gain/(Loss) on Sale is net of closing and other transaction related costs.

[2] The Company recorded an impairment loss of $0.8 million for 100 Gateway Centre Parkway during the second quarter of 2015. As such, there was no gain/(loss) at disposition for this property.

[3] The Company recorded an impairment loss of $1.7 million for Lake Merritt Tower at March 31, 2015. As such, there was no gain/(loss) at disposition for this property. Sales proceeds were deposited in escrow under Section 1031 of the Internal Revenue Code and applied to purchase the Broadmoor Austin portfolio. Refer to Broadmoor Austin Associates acquisition summary, above, for further details.