Segment Information - Performance of the Reportable Segments Based on Property Level Operating Income (Details) (USD $)	3 Months Ended									12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011		Mar. 31, 2011	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
Segment Reporting Information [Line Items]
Segment Reporting Information, Intersegment Revenue										$ 0
Operating revenue	176,227,000	171,727,000	168,567,000	169,031,000	168,704,000	165,192,000	163,948,000	[1]	165,397,000	685,552,000		663,241,000		652,009,000
Interest expense										(119,630,000)		(120,718,000)		(132,951,000)
Equity in (loss) earnings of unconsolidated joint ventures										(681,000)		3,496,000		2,296,000
Income from continuing operations	32,178,000	31,742,000	34,460,000	35,450,000	34,387,000	35,889,000	38,987,000	[1]	30,798,000	133,830,000		140,061,000		130,140,000
Northeast Southeastern PA [Member]
Segment Reporting Information [Line Items]
Operating revenue										169,645,000		174,063,000		181,412,000
Northeast Southeastern PA [Member] | Segment, Geographical, Domestic, United States of America [Member]
Segment Reporting Information [Line Items]
Operating revenue										169,645,000		174,063,000		181,412,000
Net Operating Income										98,729,000		101,982,000		108,593,000
Northeast Lehigh and Central PA [Member]
Segment Reporting Information [Line Items]
Operating revenue										96,804,000		99,311,000		103,802,000
Northeast Lehigh and Central PA [Member] | Segment, Geographical, Domestic, United States of America [Member]
Segment Reporting Information [Line Items]
Operating revenue										96,804,000		99,311,000		103,802,000
Net Operating Income										65,566,000		64,786,000		66,632,000
Northeast Other [Member]
Segment Reporting Information [Line Items]
Operating revenue										62,823,000		70,582,000		74,660,000
Northeast Other [Member] | Segment, Geographical, Domestic, United States of America [Member]
Segment Reporting Information [Line Items]
Operating revenue										62,823,000		70,582,000		74,660,000
Net Operating Income										31,711,000		35,017,000		37,234,000
Central Region [Member]
Segment Reporting Information [Line Items]
Operating revenue										120,316,000		126,268,000		125,501,000
Central Region [Member] | Segment, Geographical, Domestic, United States of America [Member]
Segment Reporting Information [Line Items]
Operating revenue										120,316,000		126,268,000		125,501,000
Net Operating Income										63,765,000		68,114,000		70,386,000
South Region [Member]
Segment Reporting Information [Line Items]
Operating revenue										209,656,000		222,004,000		232,071,000
South Region [Member] | Segment, Geographical, Domestic, United States of America [Member]
Segment Reporting Information [Line Items]
Operating revenue										209,656,000		222,004,000		232,071,000
Net Operating Income										126,281,000		132,703,000		136,135,000
Metro [Member]
Segment Reporting Information [Line Items]
Operating revenue										33,360,000		28,578,000		28,617,000
Metro [Member] | Segment, Geographical, Domestic, United States of America [Member]
Segment Reporting Information [Line Items]
Operating revenue										33,360,000		28,578,000		28,617,000
Net Operating Income										23,435,000		19,370,000		20,713,000
United Kingdom [Member]
Segment Reporting Information [Line Items]
Operating revenue										4,694,000		4,408,000		4,211,000
United Kingdom [Member] | Segment, Geographical, Foreign, United Kingdom [Member]
Segment Reporting Information [Line Items]
Operating revenue										4,694,000		4,408,000		4,211,000
Net Operating Income										(257,000)		(178,000)		243,000
Operating Segments [Member]
Segment Reporting Information [Line Items]
Operating revenue										697,298,000		725,214,000		750,274,000
Discontinued Operations [Member]
Segment Reporting Information [Line Items]
Operating revenue										(12,203,000)		(62,181,000)		(98,090,000)
Significant Reconciling Items [Member]
Segment Reporting Information [Line Items]
Net Operating Income										409,230,000		421,794,000		439,936,000
Interest expense										(123,146,000)	[2]	(131,046,000)	[2]	(149,704,000)	[2]
Depreciation and amortization										(104,643,000)	[3]	(106,487,000)	[3]	(109,265,000)	[3]
Gain on property dispositions										4,123,000		5,025,000		4,238,000
Equity in (loss) earnings of unconsolidated joint ventures										(681,000)		3,496,000		2,296,000
General and administrative										(40,831,000)	[3]	(36,140,000)	[3]	(32,805,000)	[3]
Discontinued Operations excluding gain on property dispositions										(2,538,000)		(10,067,000)		(16,378,000)
Income taxes										(874,000)	[3]	(841,000)	[3]	(1,606,000)	[3]
Other reconciling items to Total Income before income taxes										(6,810,000)		(5,673,000)		(6,572,000)
Corporate [Member]
Segment Reporting Information [Line Items]
Operating revenue										$ 457,000		$ 208,000		$ (175,000)

[1] Includes gain on sale included in discontinued operations of $50.2 million.

[2] Includes interest on discontinued operations.

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