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Fair Value of Financial Instruments and Concentrations of Credit Risk - Accounts Receivable (Details) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Fair Value Disclosures [Abstract]    
Gross trade receivables $ 256,851 $ 162,569
Allowance for doubtful accounts (9,457) (8,849)
Net trade receivables 247,394 153,720
Income tax receivables 6,905 39,944
Other receivables 11,567 20,643
Total receivables, net $ 265,866 $ 214,307