| 
           Organization (Details) 
         | 
        6 Months Ended | 
|---|---|
| 
           Jun. 30, 2011 
         | 
      |
| 
           Wholly owned operating office properties 
         | 
        |
| Investments in real estate | |
| Number of real estate properties | 249 | 
| Square footage of real estate properties (in square feet) | 20,200,000 | 
| 
           Wholly owned office properties under construction, development or redevelopment 
         | 
        |
| Investments in real estate | |
| Number of real estate properties | 17 | 
| Square footage of real estate properties (in square feet) | 2,200,000 | 
| 
           Wholly owned properties under construction, development or redevelopment which are partially operational. 
         | 
        |
| Investments in real estate | |
| Number of real estate properties | 2 | 
| 
           Wholly owned land parcels 
         | 
        |
| Investments in real estate | |
| Developable square footage of real estate properties (in square feet) | 13,000,000 | 
| Acres of real estate property (in acres) | 1,516 | 
| 
           Wholly owned, partially operational, wholesale data center 
         | 
        |
| Investments in real estate | |
| Expected initial stabilization critical load (in megawatts) | 18 | 
| X | ||||||||||
| 
             
- Definition 
          Represents the area of wholly owned land parcels in acres. 
  | 
        
| X | ||||||||||
| 
             
- Definition 
          The amount of square feet into which a land parcel is potentially developable. 
  | 
        
| X | ||||||||||
| 
             
- Definition 
          Represents expected megawatts of critical load handled by the data center upon initial stabilization. 
  | 
        
| X | ||||||||||
| 
             
- Definition 
          The number of real estate properties owned as of the balance sheet date. 
  | 
        
| X | ||||||||||
| 
             
- Definition 
          Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. 
  | 
        
| X | ||||||||||
| 
             
- Definition 
          The amount of square footage related to a real estate property. 
  |