Concentration (Details)	9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012
Concentration
Number of properties owned	81
Number of states in which acquired properties located	31
Concentration
Number of state governments	10
Annualized rental income, excluding properties classified as discontinued operations | California
Concentration
Annualized Rental income percent	0.112
Annualized rental income, excluding properties classified as discontinued operations | Maryland
Concentration
Annualized Rental income percent	0.138
Annualized rental income, excluding properties classified as discontinued operations | District of Columbia
Concentration
Annualized Rental income percent	0.108
Annualized rental income, excluding properties classified as discontinued operations | Georgia
Concentration
Annualized Rental income percent	0.097
Annualized rental income, excluding properties classified as discontinued operations | New York
Concentration
Annualized Rental income percent	0.089
Annualized rental income, excluding properties classified as discontinued operations | Massachusetts
Concentration
Annualized Rental income percent	0.060
Annualized rental income, excluding properties classified as discontinued operations | Tenant concentration | U.S. Government, state governments and the United Nations
Concentration
Concentration risk, percentage	93.90%	93.70%
Annualized rental income, excluding properties classified as discontinued operations | Tenant concentration | U.S. Government
Concentration
Concentration risk, percentage	71.20%	70.20%