Real Estate Investments Acquisitions and Dispositions (Details) (USD $)	6 Months Ended							0 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 28, 2013 The Bluffs [Member] sqft	Jun. 19, 2013 100 Applebrook [Member] sqft	Jun. 28, 2013 100 Arrandale Boulevard [Member] sqft	Jun. 14, 2013 Pacific View Plaza [Member] sqft	Feb. 25, 2013 Princeton Pike Corporate Center [Member] Properties sqft	Apr. 25, 2013 Three Logan Square [Member] acre
Significant Acquisitions and Disposals [Line Items]
Area of land acquired								0
Number of real estate properties sold							8
Net Rentable Space Sold			67,909	28,000	34,931	51,695	800,546
Sale Consideration Of Sold Property			$ 18,000,000	$ 2,700,000	$ 3,500,000	$ 10,300,000	$ 121,000,000
Lease percentage of sold property			98.80%			90.50%	86.90%
Business Acquisition, Cost of Acquired Entity, Purchase Price								20,800,000
Adjustment to Record Land Acquisition to Fair Value	(4,386,000)	0						4,300,000
Business Acquisition, Cost of Acquired Entity, Transaction Costs								$ 100,000