Segment Information - Performance of the Reportable Segments Based on Property Level Operating Income (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended				6 Months Ended
	Jun. 30, 2012		Jun. 30, 2011		Jun. 30, 2012		Jun. 30, 2011
Segment Reporting Information [Line Items]
Operating revenue	$ 169,199		$ 164,427		$ 338,886		$ 330,313
Interest expense	(30,834)		(29,358)		(60,109)		(62,200)
Equity in earnings of unconsolidated joint ventures	769		1,109		1,685		1,643
Income tax expense	(146)		(63)		(324)		(613)
Income from continuing operations	34,583		38,813		69,387		69,492
Northeast Southeastern PA [Member] | Segment, Geographical, Domestic [Member]
Segment Reporting Information [Line Items]
Operating revenue	41,690		42,809		83,885		88,073
Net Operating Income	24,814		25,571		49,984		51,095
Northeast Lehigh/Central PA [Member] | Segment, Geographical, Domestic [Member]
Segment Reporting Information [Line Items]
Operating revenue	23,913		24,739		47,991		52,939
Net Operating Income	16,558		16,417		32,753		33,866
Northeast Other [Member] | Segment, Geographical, Domestic [Member]
Segment Reporting Information [Line Items]
Operating revenue	14,454		17,436		31,854		35,729
Net Operating Income	7,628		9,063		16,733		18,258
Central Region [Member] | Segment, Geographical, Domestic [Member]
Segment Reporting Information [Line Items]
Operating revenue	28,827		31,342		60,971		63,035
Net Operating Income	15,544		17,390		33,291		34,863
South Region [Member] | Segment, Geographical, Domestic [Member]
Segment Reporting Information [Line Items]
Operating revenue	51,746		56,854		105,758		113,218
Net Operating Income	31,316		34,090		63,971		68,092
Metro [Member] | Segment, Geographical, Domestic [Member]
Segment Reporting Information [Line Items]
Operating revenue	8,236		6,951		16,424		14,100
Net Operating Income	5,986		5,044		11,615		11,001
United Kingdom [Member] | Statement, Geographical, Foreign [Member]
Segment Reporting Information [Line Items]
Operating revenue	1,104		1,083		2,265		2,213
Net Operating Income	(121)		(234)		(349)		(284)
Operating Segments [Member]
Segment Reporting Information [Line Items]
Operating revenue	169,970		181,214		349,148		369,307
Net Operating Income	101,725		107,341		207,998		216,891
Discontinued Operations [Member]
Segment Reporting Information [Line Items]
Operating revenue	576		16,886		10,235		39,095
Corporate Elimination [Member]
Segment Reporting Information [Line Items]
Operating revenue	(195)		99		(27)		101
Significant Reconciling Items [Member]
Segment Reporting Information [Line Items]
Interest expense	(30,911)	[1]	(32,203)	[1]	(61,815)	[1]	(68,372)	[1]
Depreciation and amortization	(25,812)	[2]	(25,677)	[2]	(51,464)	[2]	(54,083)	[2]
Gain on property dispositions	335		302		858		1,463
Equity in earnings of unconsolidated joint ventures	769		1,109		1,685		1,643
General and administrative	(8,936)	[2]	(7,606)	[2]	(20,486)	[2]	(18,055)	[2]
Discontinued Operations excluding gain on property dispositions	(116)		(3,871)		(3,850)		(7,665)
Income tax expense	(146)		(63)		(324)		(613)
Other reconciling items to income from continuing operations	$ 2,325		$ 519		$ 3,215		$ 1,717

[1] Includes interest on discontinued operations.

[2] Excludes costs which are included in determining segment-level net operating income.During the six months ended June 30, 2012, the Company realized proceeds of $215.1 million from the sale of 56 operating properties and 58 acres of land. The Company's total assets by reportable segment as of June 30, 2012 and December 31, 2011 are as follows (in thousands): As of June 30, 2012 December 31, 2011Total assets Northeast - Southeastern PA$828,053 $842,779 Northeast - Lehigh / Central PA758,785 716,772 Northeast - Other380,854 424,005 Central939,847 991,776 South1,363,067 1,448,849 Metro435,500 383,725 United Kingdom141,586 144,558 Other160,620 37,209Total assets$5,008,312 $4,989,673