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Accounting Policies - Revenue Recognition and Accounts Receivable (Details)
$ in Millions
12 Months Ended
Dec. 31, 2017
USD ($)
customer
Dec. 31, 2016
USD ($)
Dec. 31, 2015
USD ($)
Accounts, Notes, Loans and Financing Receivable [Line Items]      
Allowances and doubtful accounts | $ $ (1.9) $ (1.5)  
Provision for Doubtful Accounts | $ $ 0.0 $ 0.0 $ 0.0
Net sales | Customer concentration risk | Largest customer      
Accounts, Notes, Loans and Financing Receivable [Line Items]      
Number of major customers | customer 1    
Percentage of total net sales 10.00% 9.00% 9.00%
Accounts receivable | Customer concentration risk      
Accounts, Notes, Loans and Financing Receivable [Line Items]      
Number of major customers | customer 1