Segment Information Segment Information - Total Assets by Segment (Details) (USD $)	3 Months Ended
	Mar. 31, 2013 properties	Dec. 31, 2012
Segment Reporting, Asset Reconciling Item [Line Items]
Proceeds from Sale of Real Estate	$ 74,700,000
Assets	5,146,673,000	5,174,179,000
Number of Real Estate Properties Sold	1
Northeast Southeastern PA [Member]
Segment Reporting, Asset Reconciling Item [Line Items]
Assets	810,493,000	816,437,000
Northeast Lehigh Central PA [Member]
Segment Reporting, Asset Reconciling Item [Line Items]
Assets	777,171,000	780,182,000
Northeast Other [Member]
Segment Reporting, Asset Reconciling Item [Line Items]
Assets	385,243,000	388,446,000
Central Region [Member]
Segment Reporting, Asset Reconciling Item [Line Items]
Assets	1,092,854,000	1,073,631,000
South Region [Member]
Segment Reporting, Asset Reconciling Item [Line Items]
Assets	1,451,573,000	1,455,805,000
Metro [Member]
Segment Reporting, Asset Reconciling Item [Line Items]
Assets	459,515,000	478,835,000
United Kingdom [Member]
Segment Reporting, Asset Reconciling Item [Line Items]
Assets	121,262,000	138,025,000
Other [Member]
Segment Reporting, Asset Reconciling Item [Line Items]
Assets	$ 48,562,000	$ 42,818,000