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Concentration of Rental Revenue (Details)
12 Months Ended
Dec. 31, 2011
region
Dec. 31, 2011
Rental revenue
Tenants
tenant
Dec. 31, 2011
Rental revenue
Tenants
United States Government
Dec. 31, 2010
Rental revenue
Tenants
United States Government
Dec. 31, 2009
Rental revenue
Tenants
United States Government
Dec. 31, 2011
Rental revenue
Tenants
Northrop Grumman Corporation
Dec. 31, 2010
Rental revenue
Tenants
Northrop Grumman Corporation
Dec. 31, 2009
Rental revenue
Tenants
Northrop Grumman Corporation
Dec. 31, 2011
Rental revenue
Tenants
Booz Allen Hamilton, Inc.
Dec. 31, 2010
Rental revenue
Tenants
Booz Allen Hamilton, Inc.
Dec. 31, 2009
Rental revenue
Tenants
Booz Allen Hamilton, Inc.
Dec. 31, 2011
Rental revenue
Tenants
Five largest tenants
Dec. 31, 2010
Rental revenue
Tenants
Five largest tenants
Dec. 31, 2009
Rental revenue
Tenants
Five largest tenants
Dec. 31, 2011
Construction contract revenue
United States Government
Dec. 31, 2010
Construction contract revenue
United States Government
Dec. 31, 2009
Construction contract revenue
United States Government
Concentration Risk                                  
Percentage required for qualification as major customer   5.00%                              
Percentage of revenue     17.00% 16.00% 15.00% 8.00% 9.00% 8.00% 6.00% 5.00% 6.00% 38.00% 35.00% 34.00% 90.00% 90.00% 90.00%
Number of tenants from which the entity has recognized the most rental revenue aggregated for revenue concentration disclosure   5                              
Number of regions with real estate operations which are not located in the Mid-Atlantic region of the United States 2