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Trade Receivables - Net Book Value (Details) - USD ($)
$ in Thousands
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Receivables [Abstract]        
Trade accounts receivables $ 499,819 $ 504,919    
(Less) Allowance for doubtful accounts (25,918) (20,818) $ (11,598) $ (6,264)
Net book value at end of period $ 473,901 $ 484,101