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Consolidated Balance Sheets (Parenthetical) - USD ($)
shares in Millions, $ in Millions
Dec. 31, 2017
Dec. 31, 2016
Statement of Financial Position [Abstract]    
Allowance for doubtful accounts $ 1.1 $ 0.8
Common stock, par value (usd per share) $ 1 $ 1
Common stock, shares authorized 90.0 90.0
Common stock, shares issued 66.1 65.4
Treasury stock, shares 6.1 5.8