Segment Information (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012 Reportable_Segments	Jun. 30, 2011	Dec. 31, 2011
Segment Reporting Information [Line Items]
Number of reportable segments			7
Real estate investments, at cost:
Operating properties	$ 4,639,047		$ 4,639,047		$ 4,793,080
Construction-in-progress	57,420		57,420		25,083
Land inventory	109,564		109,564		109,008
Total revenue	137,576	139,249	276,987	280,866
Property operating expenses, real estate taxes and third party management expenses	53,304	54,407	108,105	113,746
Net operating income	84,272	84,842	168,882	167,120
Pennsylvania Suburbs [Member]
Real estate investments, at cost:
Operating properties	1,167,390		1,167,390		1,218,071
Total revenue	36,934	37,159	75,039	75,136
Property operating expenses, real estate taxes and third party management expenses	12,711	12,825	26,055	28,617
Net operating income	24,223	24,334	48,984	46,519
Philadelphia CBD [Member]
Real estate investments, at cost:
Operating properties	955,563		955,563		953,870
Total revenue	31,911	30,573	65,061	62,423
Property operating expenses, real estate taxes and third party management expenses	11,965	11,203	24,650	23,423
Net operating income	19,946	19,370	40,411	39,000
Metropolitan, D.C. [Member]
Real estate investments, at cost:
Operating properties	1,182,895		1,182,895		1,255,803
Total revenue	26,967	30,309	53,403	60,640
Property operating expenses, real estate taxes and third party management expenses	10,454	11,486	20,520	23,135
Net operating income	16,513	18,823	32,883	37,505
New Jersey/ Delaware [Member]
Real estate investments, at cost:
Operating properties	545,241		545,241		545,657
Total revenue	20,287	19,832	40,889	39,811
Property operating expenses, real estate taxes and third party management expenses	8,932	9,516	18,668	20,603
Net operating income	11,355	10,316	22,221	19,208
Richmond, Virginia [Member]
Real estate investments, at cost:
Operating properties	307,932		307,932		307,698
Total revenue	8,902	9,278	17,937	18,076
Property operating expenses, real estate taxes and third party management expenses	3,471	3,614	7,113	7,003
Net operating income	5,431	5,664	10,824	11,073
Austin, Texas [Member]
Real estate investments, at cost:
Operating properties	258,924		258,924		257,694
Total revenue	8,188	7,637	15,681	15,749
Property operating expenses, real estate taxes and third party management expenses	3,612	3,412	7,009	6,506
Net operating income	4,576	4,225	8,672	9,243
California [Member]
Real estate investments, at cost:
Operating properties	221,102		221,102		254,287
Total revenue	4,831	4,781	9,800	9,584
Property operating expenses, real estate taxes and third party management expenses	2,488	2,563	4,858	5,004
Net operating income	2,343	2,218	4,942	4,580
Corporate [Member]
Real estate investments, at cost:
Operating properties	0		0		0
Construction-in-progress	57,420		57,420		25,083
Land inventory	109,564		109,564		109,008
Total revenue	(444)	(320)	(823)	(553)
Property operating expenses, real estate taxes and third party management expenses	(329)	(212)	(768)	(545)
Net operating income	$ (115)	$ (108)	$ (55)	$ (8)