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Commitments and Contingencies (Details) (USD $)
 12 Months Ended
Dec. 31, 2013
Dec. 31, 2012
Dec. 31, 2011
Surety Bonds      
Outstanding bonds $ 34,100,000 $ 34,300,000  
Future minimum lease payments under non-cancelable operating leases      
2014 31,649,000    
2015 30,406,000    
2016 24,156,000    
2017 18,874,000    
2018 15,385,000    
Thereafter 40,402,000    
Total future minimum lease payment 160,872,000    
Rental expenses for properties and equipment 24,700,000 27,300,000 21,800,000
Sales and Purchase Commitments      
Sales commitments under derivative programs 265,000,000    
Purchase commitments under derivative programs $ 13,900,000    
Sales and purchase commitments period 14 days