Segment Information (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012 Reportable_Segments	Sep. 30, 2011	Dec. 31, 2011
Segment Reporting Information [Line Items]
Number of reportable segments			7
Real estate investments, at cost:
Operating properties	$ 4,660,289		$ 4,660,289		$ 4,793,080
Construction-in-progress	43,449		43,449		25,083
Land inventory	95,500		95,500		109,008
Total revenue	140,383	139,401	417,370	420,267
Property operating expenses, real estate taxes and third party management expenses	56,067	55,371	164,172	169,117
Net operating income	84,316	84,030	253,198	251,150
Pennsylvania Suburbs [Member]
Real estate investments, at cost:
Operating properties	1,174,597		1,174,597		1,218,071
Total revenue	38,526	36,812	113,565	111,949
Property operating expenses, real estate taxes and third party management expenses	13,588	13,493	39,643	42,112
Net operating income	24,938	23,319	73,922	69,837
Philadelphia CBD [Member]
Real estate investments, at cost:
Operating properties	962,235		962,235		953,870
Total revenue	33,025	31,097	98,086	93,520
Property operating expenses, real estate taxes and third party management expenses	12,431	11,539	37,082	34,962
Net operating income	20,594	19,558	61,004	58,558
Metropolitan, D.C. [Member]
Real estate investments, at cost:
Operating properties	1,189,014		1,189,014		1,255,803
Total revenue	26,516	29,296	79,919	89,936
Property operating expenses, real estate taxes and third party management expenses	10,637	10,868	31,158	34,002
Net operating income	15,879	18,428	48,761	55,934
New Jersey/ Delaware [Member]
Real estate investments, at cost:
Operating properties	545,104		545,104		545,657
Total revenue	20,553	20,743	61,442	60,554
Property operating expenses, real estate taxes and third party management expenses	10,030	10,008	28,697	30,611
Net operating income	10,523	10,735	32,745	29,943
Richmond, Virginia [Member]
Real estate investments, at cost:
Operating properties	308,371		308,371		307,698
Total revenue	8,826	9,334	26,762	27,409
Property operating expenses, real estate taxes and third party management expenses	3,762	3,664	10,875	10,666
Net operating income	5,064	5,670	15,887	16,743
Austin, Texas [Member]
Real estate investments, at cost:
Operating properties	259,304		259,304		257,694
Total revenue	8,275	7,453	23,959	23,203
Property operating expenses, real estate taxes and third party management expenses	3,484	3,233	10,497	9,740
Net operating income	4,791	4,220	13,462	13,463
California [Member]
Real estate investments, at cost:
Operating properties	221,664		221,664		254,287
Total revenue	5,141	4,968	14,942	14,551
Property operating expenses, real estate taxes and third party management expenses	2,639	3,009	7,498	8,015
Net operating income	2,502	1,959	7,444	6,536
Corporate [Member]
Real estate investments, at cost:
Operating properties	0		0		0
Construction-in-progress	43,449		43,449		25,083
Land inventory	95,500		95,500		109,008
Total revenue	(479)	(302)	(1,305)	(855)
Property operating expenses, real estate taxes and third party management expenses	(504)	(443)	(1,278)	(991)
Net operating income	25	141	(27)	136
Subsidiaries [Member]
Real estate investments, at cost:
Operating properties	4,660,289		4,660,289		4,793,080
Construction-in-progress	43,449		43,449		25,083
Land inventory	95,500		95,500		109,008
Total revenue	$ 140,383	$ 139,401	$ 417,370	$ 420,267