Segment Information Segment Information - Total Assets by Segment (Details) (USD $)	9 Months Ended
	Sep. 30, 2012 properties acre	Dec. 31, 2011
Segment Reporting, Asset Reconciling Item [Line Items]
Proceeds from Sale of Real Estate	$ 220,400,000
Number of Real Estate Properties Sold	56
Land Sold	107
Assets	4,956,365,000	4,989,673,000
Northeast Southeastern PA [Member]
Segment Reporting, Asset Reconciling Item [Line Items]
Assets	840,217,000	842,779,000
Northeast Lehigh/Central PA [Member]
Segment Reporting, Asset Reconciling Item [Line Items]
Assets	785,673,000	716,772,000
Northeast Other [Member]
Segment Reporting, Asset Reconciling Item [Line Items]
Assets	382,333,000	424,005,000
Central Region [Member]
Segment Reporting, Asset Reconciling Item [Line Items]
Assets	955,192,000	991,776,000
South Region [Member]
Segment Reporting, Asset Reconciling Item [Line Items]
Assets	1,359,110,000	1,448,849,000
Metro [Member]
Segment Reporting, Asset Reconciling Item [Line Items]
Assets	471,893,000	383,725,000
United Kingdom [Member]
Segment Reporting, Asset Reconciling Item [Line Items]
Assets	140,153,000	144,558,000
Corporate Elimination [Member]
Segment Reporting, Asset Reconciling Item [Line Items]
Assets	$ 21,794,000	$ 37,209,000