Segment Information (Performance of the Reportable Segments Based on Property Level Operating Income) (Details) (USD $) In Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010
Segment Reporting Information [Line Items]
Operating revenue	$ 175,179	$ 175,456	$ 523,966	$ 521,259
Rental property expenses and real estate taxes	56,602	56,096	164,541	165,906
Property level operating income	118,577	119,360	359,425	355,353
Interest and other income	2,019	2,400	7,011	7,748
Interest expense	(31,188)	(32,410)	(96,354)	(103,922)
General and administrative	(13,625)	(12,608)	(42,848)	(40,023)
Depreciation and amortization	(41,414)	(39,996)	(123,840)	(119,091)
Income before property dispositions, income taxes and equity in earnings of unconsolidated joint ventures	34,369	36,746	103,394	100,065
Gain on property dispositions	1,898	691	3,361	3,701
Income taxes	(356)	(475)	(969)	(1,430)
Equity in earnings of unconsolidated joint ventures	827	385	2,470	1,562
Discontinued operations	4,943	3,659	61,209	12,944
Net income	41,681	41,006	169,465	116,842
Northeast Southeastern PA [Member]
Segment Reporting Information [Line Items]
Operating revenue	42,673	45,475	130,766	135,523
Rental property expenses and real estate taxes	14,232	14,528	44,400	44,056
Property level operating income	28,441	30,947	86,366	91,467
Northeast Lehigh/Central PA [Member]
Segment Reporting Information [Line Items]
Operating revenue	22,523	21,388	67,829	61,510
Rental property expenses and real estate taxes	5,132	4,752	15,489	14,100
Property level operating income	17,391	16,636	52,340	47,410
Northeast Other [Member]
Segment Reporting Information [Line Items]
Operating revenue	17,312	18,335	52,630	55,295
Rental property expenses and real estate taxes	6,673	6,889	19,520	20,307
Property level operating income	10,639	11,446	33,110	34,988
Central Region [Member]
Segment Reporting Information [Line Items]
Operating revenue	31,055	28,856	89,186	86,047
Rental property expenses and real estate taxes	11,179	10,001	31,293	30,236
Property level operating income	19,876	18,855	57,893	55,811
South Region [Member]
Segment Reporting Information [Line Items]
Operating revenue	53,469	53,107	159,088	158,316
Rental property expenses and real estate taxes	17,562	18,010	49,140	51,936
Property level operating income	35,907	35,097	109,948	106,380
Metro [Member]
Segment Reporting Information [Line Items]
Operating revenue	7,039	7,214	21,146	21,442
Rental property expenses and real estate taxes	1,718	1,680	4,105	4,584
Property level operating income	5,321	5,534	17,041	16,858
United Kingdom [Member]
Segment Reporting Information [Line Items]
Operating revenue	1,108	1,081	3,321	3,126
Rental property expenses and real estate taxes	106	236	594	687
Property level operating income	$ 1,002	$ 845	$ 2,727	$ 2,439