Real Estate Properties Sold (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2011 sqft	Dec. 31, 2010 sqft
Real Estate Properties [Line Items]
Number of Real Estate Properties Sold	62	10
Sold Real Estate Property, Square Feet	4,219,380	678,327
Proceeds from Sale of Real Estate	$ 365,190	$ 29,016
Northeast Southeastern PA [Member]
Real Estate Properties [Line Items]
Number of Real Estate Properties Sold	1	2
Sold Real Estate Property, Square Feet	35,212	63,925
Proceeds from Sale of Real Estate	3,882	5,987
Northeast Lehigh/Central PA [Member]
Real Estate Properties [Line Items]
Number of Real Estate Properties Sold	32	2
Sold Real Estate Property, Square Feet	1,422,501	146,800
Proceeds from Sale of Real Estate	124,000	7,216
Northeast Other [Member]
Real Estate Properties [Line Items]
Number of Real Estate Properties Sold	2	1
Sold Real Estate Property, Square Feet	91,698	39,151
Proceeds from Sale of Real Estate	11,351	5,100
Central Region [Member]
Real Estate Properties [Line Items]
Number of Real Estate Properties Sold	6	1
Sold Real Estate Property, Square Feet	919,480	26,660
Proceeds from Sale of Real Estate	71,301	523
South Region [Member]
Real Estate Properties [Line Items]
Number of Real Estate Properties Sold	21	4
Sold Real Estate Property, Square Feet	1,750,489	401,791
Proceeds from Sale of Real Estate	$ 154,656	$ 10,190