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Revenue - Receivables (Details) - USD ($)
$ in Thousands
Sep. 30, 2018
Jan. 01, 2018
Accounts, Notes, Loans and Financing Receivable [Line Items]    
Total customer receivables $ 326,841 $ 297,796
Unbilled receivables 14,435 11,996
Billed receivables    
Accounts, Notes, Loans and Financing Receivable [Line Items]    
Total customer receivables $ 312,406 $ 285,800