Segment Information Segment Information - Total Assets by Segment (Details) (USD $)	6 Months Ended
	Jun. 30, 2012 properties acre	Dec. 31, 2011
Segment Reporting, Asset Reconciling Item [Line Items]
Proceeds from Sale of Real Estate	$ 215,100,000
Assets	5,008,312,000	4,989,673,000
Number of Real Estate Properties Sold	56
number of acres of land sold	58
Northeast Southeastern PA [Member]
Segment Reporting, Asset Reconciling Item [Line Items]
Assets	828,053,000	842,779,000
Northeast Lehigh/Central PA [Member]
Segment Reporting, Asset Reconciling Item [Line Items]
Assets	758,785,000	716,772,000
Northeast Other [Member]
Segment Reporting, Asset Reconciling Item [Line Items]
Assets	380,854,000	424,005,000
Central Region [Member]
Segment Reporting, Asset Reconciling Item [Line Items]
Assets	939,847,000	991,776,000
South Region [Member]
Segment Reporting, Asset Reconciling Item [Line Items]
Assets	1,363,067,000	1,448,849,000
Metro [Member]
Segment Reporting, Asset Reconciling Item [Line Items]
Assets	435,500,000	383,725,000
United Kingdom [Member]
Segment Reporting, Asset Reconciling Item [Line Items]
Assets	141,586,000	144,558,000
Corporate [Member]
Segment Reporting, Asset Reconciling Item [Line Items]
Assets	$ 160,620,000	$ 37,209,000