Segment Information - Real Estate Assets by Segment (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010
Segment Reporting Information [Line Items]
Real Estate Investment Property, Net	$ 4,219,955	$ 3,969,929
Northeast Southeastern PA [Member]
Segment Reporting Information [Line Items]
Real Estate Investment Property, Net	780,228	797,071
Northeast Lehigh/Central PA [Member]
Segment Reporting Information [Line Items]
Real Estate Investment Property, Net	677,519	632,399
Northeast Other [Member]
Segment Reporting Information [Line Items]
Real Estate Investment Property, Net	360,043	369,172
Central Region [Member]
Segment Reporting Information [Line Items]
Real Estate Investment Property, Net	843,844	756,553
South Region [Member]
Segment Reporting Information [Line Items]
Real Estate Investment Property, Net	1,288,907	1,232,983
Metro [Member]
Segment Reporting Information [Line Items]
Real Estate Investment Property, Net	227,931	138,220
United Kingdom [Member]
Segment Reporting Information [Line Items]
Real Estate Investment Property, Net	$ 41,483	$ 43,531