Accounting for the Impairment or Disposal of Long-Lived Assets - Real Estate Impairments in Reportable Segments (Details) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011		Dec. 31, 2010
Schedule of Real Estate Impairment [Line Items]
Asset Impairment Charges including amounts in Discontinued Operations	$ 6,902,000	$ 7,829,000		$ 957,000
Northeast Southeastern PA [Member]
Schedule of Real Estate Impairment [Line Items]
Asset Impairment Charges including amounts in Discontinued Operations	1,699,000	0		(52,000)	[1]
Northeast Lehigh and Central PA [Member]
Schedule of Real Estate Impairment [Line Items]
Asset Impairment Charges including amounts in Discontinued Operations	0	0		0
Northeast Other [Member]
Schedule of Real Estate Impairment [Line Items]
Asset Impairment Charges including amounts in Discontinued Operations	29,000	538,000		0
Central Region [Member]
Schedule of Real Estate Impairment [Line Items]
Asset Impairment Charges including amounts in Discontinued Operations	566,000	5,990,000		511,000
South Region [Member]
Schedule of Real Estate Impairment [Line Items]
Asset Impairment Charges including amounts in Discontinued Operations	11,000	1,331,000		121,000
Metro [Member]
Schedule of Real Estate Impairment [Line Items]
Asset Impairment Charges including amounts in Discontinued Operations	0	(30,000)	[1]	377,000
United Kingdom [Member]
Schedule of Real Estate Impairment [Line Items]
Asset Impairment Charges including amounts in Discontinued Operations	$ 4,597,000	$ 0		$ 0

[1] Represents recovery of estimated sales costs on properties sold.