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Short-Term Loans (Details) (USD $)
3 Months Ended 6 Months Ended
Jun. 30, 2011
Jun. 30, 2010
Jun. 30, 2011
Jun. 30, 2010
Dec. 31, 2010
Short-Term Loans          
Irrevocable letter of credit outstanding amount at the end of period         $ 23,500,000
Revolving credit facility outstanding amount at the end of period         53,650,000
Commitment fee paid to maintain credit facility 5.0 basis points 2.0 basis points      
Maximum amount of outstanding short-term loans at any time during the period     104,000,000 139,700,000  
Average daily balance of short-term loans outstanding during the period     $ 11,100,000 $ 26,600,000  
Weighted average annual interest rates of average daily balance of short-term loans (as a percent)     1.81% 0.86%