Segment Information (Details) (USD $) In Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010	Dec. 31, 2010
Real estate investments, at cost:
Operating properties	$ 4,920,728		$ 4,920,728		$ 4,834,111
Construction-in-progress	36,246		36,246		33,322
Land inventory	120,470		120,470		110,055
Total revenue	145,664	141,492	439,275	417,989
Property operating expenses, real estate taxes and third party management expenses	57,065	57,475	176,106	168,837
Net operating income	88,059	84,017	263,169	249,152
Pennsylvania Suburbs [Member]
Real estate investments, at cost:
Operating properties	1,214,060		1,214,060		1,199,957
Construction-in-progress	0		0		0
Land inventory	0		0		0
Total revenue	38,864	36,809	118,295	113,622
Property operating expenses, real estate taxes and third party management expenses	14,224	14,591	44,554	44,218
Net operating income	24,640	22,218	73,741	69,404
Philadelphia CBD [Member]
Real estate investments, at cost:
Operating properties	951,790		951,790		911,354
Construction-in-progress	0		0		0
Land inventory	0		0		0
Total revenue	31,098	24,617	93,520	61,579
Property operating expenses, real estate taxes and third party management expenses	11,541	9,859	34,954	25,643
Net operating income	19,557	14,758	58,566	35,936
Metropolitan, D.C. [Member]
Real estate investments, at cost:
Operating properties	1,369,899		1,369,899		1,359,776
Construction-in-progress	0		0		0
Land inventory	0		0		0
Total revenue	31,554	33,881	96,813	103,285
Property operating expenses, real estate taxes and third party management expenses	11,526	11,665	36,012	36,041
Net operating income	20,028	22,216	60,801	67,244
New Jersey/ Delaware [Member]
Real estate investments, at cost:
Operating properties	570,804		570,804		568,413
Construction-in-progress	0		0		0
Land inventory	0		0		0
Total revenue	22,096	23,994	64,526	71,884
Property operating expenses, real estate taxes and third party management expenses	10,576	11,930	32,302	34,716
Net operating income	11,520	12,014	32,224	37,168
Richmond, Virginia [Member]
Real estate investments, at cost:
Operating properties	306,039		306,039		294,406
Construction-in-progress	0		0		0
Land inventory	0		0		0
Total revenue	9,334	8,919	27,409	27,087
Property operating expenses, real estate taxes and third party management expenses	3,664	3,287	10,667	10,111
Net operating income	5,670	5,632	16,742	16,976
Austin, Texas [Member]
Real estate investments, at cost:
Operating properties	257,208		257,208		254,019
Construction-in-progress	0		0		0
Land inventory	0		0		0
Total revenue	7,453	8,014	23,202	23,979
Property operating expenses, real estate taxes and third party management expenses	3,233	3,204	9,740	9,968
Net operating income	4,220	4,810	13,462	14,011
Corporate [Member]
Real estate investments, at cost:
Operating properties	0		0		0
Construction-in-progress	36,246		36,246		33,322
Land inventory	120,470		120,470		110,055
Total revenue	(302)	(195)	(849)	(821)
Property operating expenses, real estate taxes and third party management expenses	(421)	(232)	(895)	(713)
Net operating income	119	37	46	(108)
California [Member]
Real estate investments, at cost:
Operating properties	250,928		250,928		246,186
Construction-in-progress	0		0		0
Land inventory	0		0		0
Total revenue	5,567	5,503	16,359	17,374
Property operating expenses, real estate taxes and third party management expenses	3,262	3,171	8,772	8,853
Net operating income	$ 2,305	$ 2,332	$ 7,587	$ 8,521