Segment Information - Performance of the Reportable Segments Based on Property Level Operating Income (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended				9 Months Ended
	Sep. 30, 2012		Sep. 30, 2011		Sep. 30, 2012		Sep. 30, 2011
Segment Reporting Information [Line Items]
Operating revenue	$ 171,726		$ 165,192		$ 509,326		$ 494,537
Reconciliation of Operating Profit (Loss) from Segments to Consolidated [Abstract]
Interest expense	(30,704)		(29,173)		(89,892)		(90,198)
Equity in (loss) earnings of unconsolidated joint ventures	(3,082)		827		(1,397)		2,470
Income from continuing operations	31,785		36,207		101,693		106,750
Northeast Southeastern PA [Member] | Segment, Geographical, Domestic [Member]
Segment Reporting Information [Line Items]
Operating revenue	42,773		42,695		126,658		130,769
Net Operating Income	24,880		25,602		74,864		76,697
Northeast Lehigh/Central PA [Member] | Segment, Geographical, Domestic [Member]
Segment Reporting Information [Line Items]
Operating revenue	24,263		22,580		72,254		75,520
Net Operating Income	16,578		15,119		49,331		48,985
Northeast Other [Member] | Segment, Geographical, Domestic [Member]
Segment Reporting Information [Line Items]
Operating revenue	15,042		17,344		46,895		53,073
Net Operating Income	7,640		8,478		24,373		26,736
Central Region [Member] | Segment, Geographical, Domestic [Member]
Segment Reporting Information [Line Items]
Operating revenue	28,969		31,566		89,940		94,601
Net Operating Income	15,432		16,650		48,723		51,512
South Region [Member] | Segment, Geographical, Domestic [Member]
Segment Reporting Information [Line Items]
Operating revenue	51,603		54,879		157,362		168,097
Net Operating Income	30,870		32,264		94,840		100,356
Metro [Member] | Segment, Geographical, Domestic [Member]
Segment Reporting Information [Line Items]
Operating revenue	8,248		7,047		24,672		21,147
Net Operating Income	5,845		4,496		17,460		15,497
United Kingdom [Member] | Statement, Geographical, Foreign [Member]
Segment Reporting Information [Line Items]
Operating revenue	1,264		1,109		3,529		3,321
Net Operating Income	171		19		(178)		(265)
Operating Segments [Member]
Segment Reporting Information [Line Items]
Operating revenue	172,162		177,220		521,310		546,528
Net Operating Income	101,416		102,628		309,413		319,518
Discontinued Operations [Member]
Segment Reporting Information [Line Items]
Operating revenue	635		11,915		12,156		51,978
Corporate Elimination [Member]
Segment Reporting Information [Line Items]
Operating revenue	199		(113)		172		(13)
Significant Reconciling Items [Member]
Reconciliation of Operating Profit (Loss) from Segments to Consolidated [Abstract]
Interest expense	(31,305)	[1]	(31,526)	[1]	(93,120)	[1]	(99,898)	[1]
Depreciation and amortization	(25,753)	[2]	(25,915)	[2]	(77,217)	[2]	(79,998)	[2]
Gain on property dispositions	2,001		1,898		2,859		3,361
Equity in (loss) earnings of unconsolidated joint ventures	(3,082)		827		(1,397)		2,470
General and administrative	(9,555)	[2]	(8,034)	[2]	(30,041)	[2]	(26,089)	[2]
Discontinued Operations excluding gain on property dispositions	347		(1,379)		(2,982)		(7,993)
Income taxes, corporate level	270	[2]	325	[2]	465	[2]	814	[2]
Other reconciling items to income from continuing operations	$ 2,014		$ 1,967		$ 5,357		$ 3,807

[1] Includes interest on discontinued operations.

[2] Excludes costs which are included in determining segment-level net operating income.