Tenant Concentration (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Tenant Concentration
Concentration risk, percentage	3.00%								3.00%
Revenue	$ 241,552	$ 241,785	$ 217,938	$ 210,673	$ 200,645	$ 196,272	$ 192,930	$ 192,616	$ 911,948	$ 782,463	$ 764,461
Revenues | Tenant concentration | Maximum
Tenant Concentration
Concentration risk, percentage	5.00%				5.00%				5.00%	5.00%
Revenues | Tenant concentration | United States Government
Tenant Concentration
Revenue											$ 78,534
Revenues | Tenant concentration | United States Government | Minimum
Tenant Concentration
Concentration risk, percentage											5.00%