Segment Information - Real Estate Assets by Segment (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Segment Reporting Information [Line Items]
Real Estate Investment Property, Net	$ 4,590,830	$ 4,205,728
Northeast Southeastern PA [Member]
Segment Reporting Information [Line Items]
Real Estate Investment Property, Net	752,423	771,022
Northeast Lehigh and Central PA [Member]
Segment Reporting Information [Line Items]
Real Estate Investment Property, Net	742,464	677,519
Northeast Other [Member]
Segment Reporting Information [Line Items]
Real Estate Investment Property, Net	358,957	355,022
Central Region [Member]
Segment Reporting Information [Line Items]
Real Estate Investment Property, Net	1,002,663	843,844
South Region [Member]
Segment Reporting Information [Line Items]
Real Estate Investment Property, Net	1,375,498	1,288,907
Metro [Member]
Segment Reporting Information [Line Items]
Real Estate Investment Property, Net	313,964	227,931
United Kingdom [Member]
Segment Reporting Information [Line Items]
Real Estate Investment Property, Net	$ 44,861	$ 41,483