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Unsecured Credit Facility (Details) (USD $)
6 Months Ended
Jun. 30, 2011
Dec. 31, 2010
Oct. 02, 2010
Unsecured Credit Facility      
Unsecured credit facility     $ 75,000,000
Available additional borrowing capacity 25,000,000    
Credit facility expiration date October 1, 2013    
Credit facility outstanding amount 0 0  
Credit facility expenses $ 300,000