Discontinued Operations (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended		0 Months Ended	12 Months Ended	3 Months Ended	9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013	Apr. 07, 2014 Princeton Pike Corporate Center [Member]	Feb. 25, 2014 Princeton Pike Corporate Center [Member]	Sep. 30, 2013 Discontinued Operations [Member]	Sep. 30, 2013 Discontinued Operations [Member]
Revenue:
Rents	$ 120,288	$ 116,382	$ 363,581	$ 347,054			$ 96	$ 4,754
Termination fees	1,418	2,040	6,970	2,946			0	0
Other Real Estate Revenue	825	975	2,295	3,305			0	123
Tenant reimbursements	20,095	20,694	64,057	60,595			10	355
Total revenue	146,558	143,354	449,172	423,552			106	5,232
Expenses:
Property operating expenses	42,675	40,639	132,612	119,421			62	1,773
Real estate taxes	12,869	14,167	39,167	42,639			39	623
Depreciation and amortization	52,616	50,583	157,773	149,300			48	1,893
Total operating expenses	115,747	113,142	354,771	335,788			149	4,289
Interest income	528	268	1,298	448			0	3
Income from discontinued operations before gain on sale of interests in real estate	0	(43)	18	946			(43)	946
Net gain on disposition of discontinued operations	(3)	(16)	900	3,029	900	5,300	(16)	3,029
Income (Loss) from discontinued operations	$ (3)	$ (59)	$ 918	$ 3,975			$ (59)	$ 3,975