Real Estate Properties (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended	6 Months Ended		1 Months Ended
	Jun. 30, 2013 sqft property	Jun. 30, 2013 property sqft	Dec. 31, 2012	Feb. 28, 2013 Oklahoma City, OK Sale of properties sqft	Mar. 31, 2013 Tucson, AZ Sale of properties sqft	Jul. 31, 2013 Chester, Virginia Warehouse property Subsequent event sqft	Jul. 31, 2013 Chester, Virginia Warehouse property U.S. Government and occupied by the United States Army Subsequent event
Real estate properties
Number of properties owned	82	82
Aggregate investment in properties	$ 1,526,678	$ 1,526,678
Square Feet	314,527	513,045				228,108
Percentage of property leased							100.00%
Aggregate purchase price of properties acquired, excluding closing costs						12,750
Number of leases entered	16	22
Expenditures committed on leases	5,191	9,802
Weighted average lease term	13 years 4 months 24 days	9 years 8 months 12 days
Committed but unspent tenant related obligations	13,600	13,600
Rentable square feet of Properties				185,881	31,051
Net book value of real estate to be sold	1,336,097	1,336,097	1,347,902	8,069	2,080
Aggregate sale price of properties sold, excluding closing costs				16,300	2,189
Gain (loss) on sale of properties				$ 8,198	$ (30)