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Accounts Receivable (Details) - USD ($)
$ in Thousands
Jun. 30, 2016
Dec. 31, 2015
Accounts Receivable, Net [Abstract]    
Trade customers $ 507,690 $ 438,538
Allowance for doubtful accounts (14,534) (14,095)
Accounts receivable from trade customers, net 493,156 424,443
Federal and state taxes 69,183 60,748
Other 20,516 23,341
Accounts receivable, net $ 582,855 $ 508,532