| 
           Properties, net (Details 4) 
         | 
        3 Months Ended | 
|---|---|
| 
           Mar. 31, 2012
 
          sqfeet
 
          property 
         | 
      |
| 
           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 | Greater Philadelphia 
         | 
        |
| Construction and Redevelopment Activities | |
| Number of real estate properties | 1 | 
| Square footage of real estate properties (in square feet) | 113,000 |