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