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General (Details 8) (USD $)
6 Months Ended
Jun. 30, 2011
Dec. 31, 2010
Apr. 28, 2010
2010 Credit Facility      
Sublimit on the amount of revolving loans     $ 30,000,000
Maximum available for the issuance of letters of credit     80,000,000
Interest rate on letters of credit issued (as a percent) 2.875%    
Commitment commission (as a percent) 0.50%    
Long Term Debt and Capital Lease Obligations      
Capital lease obligations 559,000 559,000  
Letters of credit outstanding $ 41,000,000 $ 44,900,000  
U.S. dollar denominated loans
     
Interest rate on Revolving Loan borrowings      
Borrowing margin (as a percent) 1.75%    
Base interest rate (as a percent) The higher of the prime rate or one-half of one percent in excess of the overnight "federal funds" rate    
Eurodollar denominated loans
     
Interest rate on Revolving Loan borrowings      
Borrowing margin (as a percent) 2.75%    
Base interest rate (as a percent) Eurodollar rate