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Trade Accounts Receivable (Details) - USD ($)
$ in Millions
3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Dec. 31, 2017
Trade Accounts Receivable      
Allowance for doubtful accounts $ 0.3   $ 0.4
Allowance for doubtful accounts receivable, reduction $ 0.1 $ 0.1