Concentration of Rental Revenue (Details)	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Concentration Risk
Number of regions with real estate operations which are not located in the Mid-Atlantic region of the United States	3
Rental revenue | Tenants
Concentration Risk
Percentage required for qualification as major customer	5.00%
Number of tenants from which the entity has recognized the most rental revenue aggregated for revenue concentration disclosure	5
Rental revenue | Tenants | United States Government
Concentration Risk
Percentage of revenue	18.00%		18.00%		18.00%
Rental revenue | Tenants | Northrop Grumman Corporation
Concentration Risk
Percentage of revenue	6.00%	[1]	8.00%	[1]	7.00%	[1]
Rental revenue | Tenants | Booz Allen Hamilton, Inc.
Concentration Risk
Percentage of revenue	6.00%		6.00%		6.00%
Rental revenue | Tenants | Computer Sciences Corporation
Concentration Risk
Percentage of revenue			5.00%	[1]	5.00%	[1]
Rental revenue | Tenants | Five largest tenants
Concentration Risk
Percentage of revenue	39.00%		41.00%		39.00%
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.