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Accounts Receivable (Details) - USD ($)
$ in Thousands
Mar. 31, 2018
Dec. 31, 2017
Customer Accounts Receivable and Note Receivable [Line Items]    
Customer Accounts Receivable and Note Receivable $ 24,200 $ 24,600
Accounts receivable 7,438,017 8,181,879
Allowances for doubtful accounts (59,564) (56,291)
Accounts receivable, net $ 7,378,453 $ 8,125,588