Concentration (Details)	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Concentration
Number of properties owned	68
Number of Buildings	87
Number of states in which acquired properties located	31
Concentration
Number of state governments	11
Annualized rental income, excluding properties classified as discontinued operations | Maryland
Concentration
Annualized Rental income percent	0.132
Annualized rental income, excluding properties classified as discontinued operations | California
Concentration
Annualized Rental income percent	0.115
Annualized rental income, excluding properties classified as discontinued operations | District of Columbia
Concentration
Annualized Rental income percent	0.103
Annualized rental income, excluding properties classified as discontinued operations | Georgia
Concentration
Annualized Rental income percent	0.098
Annualized rental income, excluding properties classified as discontinued operations | New York
Concentration
Annualized Rental income percent	0.088
Annualized rental income, excluding properties classified as discontinued operations | Massachusetts
Concentration
Annualized Rental income percent	0.058
Annualized rental income, excluding properties classified as discontinued operations | Tenant concentration | U.S. Government, state governments and the United Nations
Concentration
Concentration risk, percentage	92.60%	93.80%	91.70%
Annualized rental income, excluding properties classified as discontinued operations | Tenant concentration | U.S. Government
Concentration
Concentration risk, percentage	69.00%	71.00%	68.10%