Discontinued Operations (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended		0 Months Ended	12 Months Ended	3 Months Ended	6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013	Apr. 07, 2014 Princeton Pike Corporate Center [Member]	Feb. 25, 2014 Princeton Pike Corporate Center [Member]	Jun. 30, 2013 Discontinued Operations [Member]	Jun. 30, 2013 Discontinued Operations [Member]
Revenue:
Rents	$ 121,622	$ 116,064	$ 243,293	$ 230,672			$ 895	$ 4,658
Termination fees	3,349	410	5,552	906			0	0
Other Real Estate Revenue	840	1,457	1,470	2,330			9	123
Tenant reimbursements	20,502	19,560	43,962	39,901			7	345
Total revenue	150,500	140,644	302,614	280,198			911	5,126
Expenses:
Property operating expenses	43,136	39,433	89,937	78,782			330	1,711
Real estate taxes	12,841	14,177	26,298	28,472			118	584
Depreciation and amortization	52,587	49,241	105,157	98,717			337	1,845
Total operating expenses	116,299	111,550	239,024	222,646			785	4,140
Interest income	385	122	770	180			3	3
Income from discontinued operations before gain on sale of interests in real estate	26	129	18	989			129	989
Net gain on disposition of discontinued operations	903	(2,259)	903	3,045	900	5,300	(2,259)	3,045
Income (Loss) from discontinued operations	$ 929	$ (2,130)	$ 921	$ 4,034			$ (2,130)	$ 4,034