Segment Information (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2012 Reportable_Segments	Mar. 31, 2011	Dec. 31, 2011
Segment Reporting Information [Line Items]
Number of reportable segments	7
Real estate investments, at cost:
Operating properties	$ 4,717,124		$ 4,793,080
Construction-in-progress	38,442		25,083
Land inventory	109,285		109,008
Total revenue	141,801	144,298
Property operating expenses, real estate taxes and third party management expenses	55,780	60,470
Net operating income	86,021	83,828
Pennsylvania Suburbs [Member]
Real estate investments, at cost:
Operating properties	1,219,154		1,218,071
Total revenue	39,829	40,053
Property operating expenses, real estate taxes and third party management expenses	14,080	16,672
Net operating income	25,749	23,381
Philadelphia CBD [Member]
Real estate investments, at cost:
Operating properties	954,484		953,870
Total revenue	33,149	31,850
Property operating expenses, real estate taxes and third party management expenses	12,686	12,219
Net operating income	20,463	19,631
Metropolitan, D.C. [Member]
Real estate investments, at cost:
Operating properties	1,181,600		1,255,803
Total revenue	26,437	30,332
Property operating expenses, real estate taxes and third party management expenses	10,065	11,648
Net operating income	16,372	18,684
New Jersey/ Delaware [Member]
Real estate investments, at cost:
Operating properties	543,293		545,657
Total revenue	20,601	19,978
Property operating expenses, real estate taxes and third party management expenses	9,737	11,088
Net operating income	10,864	8,890
Richmond, Virginia [Member]
Real estate investments, at cost:
Operating properties	307,275		307,698
Total revenue	9,034	8,798
Property operating expenses, real estate taxes and third party management expenses	3,643	3,388
Net operating income	5,391	5,410
Austin, Texas [Member]
Real estate investments, at cost:
Operating properties	257,397		257,694
Total revenue	7,493	8,114
Property operating expenses, real estate taxes and third party management expenses	3,396	3,093
Net operating income	4,097	5,021
California [Member]
Real estate investments, at cost:
Operating properties	253,921		254,287
Total revenue	5,636	5,409
Property operating expenses, real estate taxes and third party management expenses	2,618	2,695
Net operating income	3,018	2,714
Corporate [Member]
Real estate investments, at cost:
Operating properties	0		0
Construction-in-progress	38,442		25,083
Land inventory	109,285		109,008
Total revenue	(378)	(236)
Property operating expenses, real estate taxes and third party management expenses	(445)	(333)
Net operating income	$ 67	$ 97