Annual report pursuant to Section 13 and 15(d)

Concentration of Rental Revenue (Details)

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Concentration of Rental Revenue (Details)
12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Concentration Risk      
Number of regions with real estate operations which are not located in the Mid-Atlantic region of the United States 2    
Rental revenue | Tenants
     
Concentration Risk      
Number of tenants from which the entity has recognized the most rental revenue aggregated for revenue concentration disclosure 5    
Percentage required for qualification as major customer 5.00%    
Rental revenue | Tenants | United States Government
     
Concentration Risk      
Percentage of revenue 18.00% 17.00% 16.00%
Rental revenue | Tenants | Northrop Grumman Corporation
     
Concentration Risk      
Percentage of revenue 7.00% [1] 8.00% [1] 9.00% [1]
Rental revenue | Tenants | Booz Allen Hamilton, Inc.
     
Concentration Risk      
Percentage of revenue 6.00% 6.00% 5.00%
Rental revenue | Tenants | Computer Sciences Corporation
     
Concentration Risk      
Percentage of revenue 5.00%    
Rental revenue | Tenants | Five largest tenants
     
Concentration Risk      
Percentage of revenue 39.00% 38.00% 35.00%
Construction contract revenue | United States Government
     
Concentration Risk      
Percentage of revenue 90.00% 90.00% 90.00%
[1] Includes affiliated organizations and agencies and predecessor companies.