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Commitments and Contingencies (Details) (USD $)
 12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Surety Bonds      
Outstanding bonds $ 34,300,000 $ 31,400,000  
Future minimum lease payments under non-cancelable operating leases      
2013 47,938,000    
2014 26,694,000    
2015 21,395,000    
2016 16,009,000    
2017 10,880,000    
Thereafter 31,525,000    
Total future minimum lease payment 154,441,000    
Rental expenses for properties and equipment 27,300,000 21,800,000 10,000,000
Sales and Purchase Commitments      
Sales commitments under derivative programs 440,700,000    
Purchase commitments under derivative programs $ 55,500,000    
Sales and purchase commitments period 14 days