Segment Information Segment Information (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Dec. 31, 2012
Real Estate Investments At Cost [Abstract]
Rental properties	$ 4,620,460		$ 4,620,460		$ 4,726,169
Construction-in-progress	51,260		51,260		48,950
Land inventory	94,444		94,444		102,439
Total revenue	140,919	131,803	280,742	265,593
Property operating expenses	(55,068)	(51,135)	(110,248)	(103,714)
Gross Profit	85,851	80,668	170,494	161,879
Pennsylvania [Member]
Real Estate Investments At Cost [Abstract]
Rental properties	1,198,731		1,198,731		1,178,730
Total revenue	37,276	36,784	75,461	74,736
Property operating expenses	(13,151)	(12,640)	(27,050)	(25,902)
Gross Profit	24,125	24,144	48,411	48,834
Philadelphia CBD [Member]
Real Estate Investments At Cost [Abstract]
Rental properties	1,023,619		1,023,619		988,590
Total revenue	36,142	31,911	72,067	65,062
Property operating expenses	(13,587)	(11,962)	(27,121)	(24,648)
Gross Profit	22,555	19,949	44,946	40,414
Metropolitan Dc [Member]
Real Estate Investments At Cost [Abstract]
Rental properties	1,199,885		1,199,885		1,193,200
Total revenue	28,783	26,967	56,567	53,403
Property operating expenses	(10,774)	(10,451)	(21,536)	(20,518)
Gross Profit	18,009	16,516	35,031	32,885
New Jersey/ Delaware [Member]
Real Estate Investments At Cost [Abstract]
Rental properties	408,940		408,940		546,644
Total revenue	15,676	15,284	31,278	31,013
Property operating expenses	(7,245)	(7,083)	(14,653)	(14,912)
Gross Profit	8,431	8,201	16,625	16,101
Richmond, Virginia [Member]
Real Estate Investments At Cost [Abstract]
Rental properties	310,229		310,229		309,923
Total revenue	8,954	8,903	17,719	17,937
Property operating expenses	(3,599)	(3,470)	(7,282)	(7,112)
Gross Profit	5,355	5,433	10,437	10,825
Austin, Texas [Member]
Real Estate Investments At Cost [Abstract]
Rental properties	286,808		286,808		285,346
Total revenue	9,871	8,188	19,129	15,682
Property operating expenses	(4,208)	(3,611)	(8,165)	(7,009)
Gross Profit	5,663	4,577	10,964	8,673
CALIFORNIA
Real Estate Investments At Cost [Abstract]
Rental properties	192,248		192,248		223,736
Total revenue	4,536	4,210	9,142	8,586
Property operating expenses	(2,339)	(2,249)	(4,498)	(4,384)
Gross Profit	2,197	1,961	4,644	4,202
Corporate [Member]
Real Estate Investments At Cost [Abstract]
Rental properties	0		0		0
Construction-in-progress	51,260		51,260		48,950
Land inventory	94,444		94,444		102,439
Total revenue	(319)	(444)	(621)	(826)
Property operating expenses	(165)	331	57	771
Gross Profit	$ (484)	$ (113)	$ (564)	$ (55)