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Accounts Receivable	12 Months Ended
Accounts Receivable	Dec. 31, 2017
Receivables [Abstract]
Accounts Receivable	5. Accounts Receivable An allowance for doubtful accounts is maintained for estimated losses resulting from the inability of our customers to make required payments. Accounts receivable are stated net of the allowance for doubtful accounts of $49.0 million at December 31, 2017 ($45.9 million at December 31, 2016 and $43.2 million at December 31, 2015).