Segment Information - Performance of the Reportable Segments Based on Property Level Operating Income (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013		Mar. 31, 2012
Segment Reporting Information [Line Items]
Operating revenue	$ 176,573		$ 166,942
Interest expense (1)	(31,934)		(28,193)
Equity in earnings of unconsolidated joint ventures	1,757		916
Income from continuing operations	32,063		34,215
Northeast Southeastern PA [Member] | Segment, Geographical, Domestic [Member]
Segment Reporting Information [Line Items]
Operating revenue	41,054		42,195
Net Operating Income	22,613		25,170
Northeast Lehigh Central PA [Member] | Segment, Geographical, Domestic [Member]
Segment Reporting Information [Line Items]
Operating revenue	24,677		24,077
Net Operating Income	16,668		16,195
Northeast Other [Member] | Segment, Geographical, Domestic [Member]
Segment Reporting Information [Line Items]
Operating revenue	15,394		17,400
Net Operating Income	8,322		9,105
Central Region [Member] | Segment, Geographical, Domestic [Member]
Segment Reporting Information [Line Items]
Operating revenue	32,401		32,144
Net Operating Income	18,923		17,747
South Region [Member] | Segment, Geographical, Domestic [Member]
Segment Reporting Information [Line Items]
Operating revenue	53,562		54,012
Net Operating Income	33,317		32,654
Metro [Member] | Segment, Geographical, Domestic [Member]
Segment Reporting Information [Line Items]
Operating revenue	9,763		8,188
Net Operating Income	6,796		5,629
United Kingdom [Member] | Statement, Geographical, Foreign [Member]
Segment Reporting Information [Line Items]
Operating revenue	1,100		1,162
Net Operating Income	(133)		(229)
Operating Segments [Member]
Segment Reporting Information [Line Items]
Operating revenue	177,951		179,178
Net Operating Income	106,506		106,271
Discontinued Operations [Member]
Segment Reporting Information [Line Items]
Operating revenue	(1,634)		(12,404)
Other [Member]
Segment Reporting Information [Line Items]
Operating revenue	256		168
Significant Reconciling Items [Member]
Segment Reporting Information [Line Items]
Interest expense (1)	(32,082)	[1]	(30,904)	[1]
Depreciation/amortization expense (2)	(29,307)	[2]	(25,652)	[2]
Gain on property dispositions	1,800		523
Equity in earnings of unconsolidated joint ventures	1,757		916
General and administrative expense (2)	(13,877)	[2]	(11,550)	[2]
Discontinued Operations excluding gain on property dispositions	(880)		(4,323)
Income taxes (2)	491	[2]	86	[2]
Other	$ 1,363		$ 980

[1] Includes interest on discontinued operations.

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