Segment Information - Total Assets by Segment (Details) (USD $)	Dec. 31, 2012	Dec. 31, 2011
Segment Reporting, Asset Reconciling Item [Line Items]
Assets	$ 5,177,971,000	$ 4,989,673,000
Corporate [Member]
Segment Reporting, Asset Reconciling Item [Line Items]
Assets	46,610,000	37,209,000
Segment, Geographical, Domestic, United States of America [Member] | Northeast Southeastern PA [Member]
Segment Reporting, Asset Reconciling Item [Line Items]
Assets	816,437,000	842,779,000
Segment, Geographical, Domestic, United States of America [Member] | Northeast Lehigh Central PA [Member]
Segment Reporting, Asset Reconciling Item [Line Items]
Assets	780,182,000	716,772,000
Segment, Geographical, Domestic, United States of America [Member] | Northeast Other [Member]
Segment Reporting, Asset Reconciling Item [Line Items]
Assets	388,446,000	424,005,000
Segment, Geographical, Domestic, United States of America [Member] | Central Region [Member]
Segment Reporting, Asset Reconciling Item [Line Items]
Assets	1,073,631,000	991,776,000
Segment, Geographical, Domestic, United States of America [Member] | South Region [Member]
Segment Reporting, Asset Reconciling Item [Line Items]
Assets	1,455,805,000	1,448,849,000
Segment, Geographical, Domestic, United States of America [Member] | Metro [Member]
Segment Reporting, Asset Reconciling Item [Line Items]
Assets	478,835,000	383,725,000
Segment, Geographical, Foreign, United Kingdom [Member] | United Kingdom [Member]
Segment Reporting, Asset Reconciling Item [Line Items]
Assets	$ 138,025,000	$ 144,558,000