Properties, net (Details 6)	12 Months Ended
	Dec. 31, 2011 property sqfeet	Dec. 31, 2010 sqfeet property
Newly-constructed properties placed in service
Construction and Redevelopment Activities
Number of real estate properties placed into service	7	9
Square feet placed in service on properties during the period (in square feet)	566,000	816,000
Newly-constructed properties placed in service | Baltimore/Washington Corridor
Construction and Redevelopment Activities
Number of real estate properties placed into service	3	3
Newly-constructed properties placed in service | Greater Baltimore
Construction and Redevelopment Activities
Number of real estate properties placed into service	2	2
Newly-constructed properties placed in service | San Antonio
Construction and Redevelopment Activities
Number of real estate properties placed into service	1	2
Newly-constructed properties placed in service | St. Mary's County
Construction and Redevelopment Activities
Number of real estate properties placed into service	1
Newly-constructed properties placed in service | Colorado Springs
Construction and Redevelopment Activities
Number of real estate properties placed into service		2
Properties under construction
Construction and Redevelopment Activities
Number of real estate properties	6
Square footage of real estate properties (in square feet)	789,000
Properties under construction | Baltimore/Washington Corridor
Construction and Redevelopment Activities
Number of real estate properties	3
Properties under construction | Greater Baltimore
Construction and Redevelopment Activities
Number of real estate properties	1
Properties under construction | Northern Virginia
Construction and Redevelopment Activities
Number of real estate properties	1
Properties under construction | Huntsville
Construction and Redevelopment Activities
Number of real estate properties	1
Properties under redevelopment
Construction and Redevelopment Activities
Square footage of real estate properties (in square feet)	113,000
Properties under redevelopment | Greater Philadelphia
Construction and Redevelopment Activities
Number of real estate properties	1