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CONSOLIDATED BALANCE SHEETS (Parenthetical) - $ / shares
shares in Millions
Dec. 31, 2017
Dec. 31, 2016
Statement of Financial Position [Abstract]    
Common stock, par value (in dollars per share) $ 0.01 $ 0.01
Common stock, shares authorized (in shares) 153.0 153.0
Common stock, shares outstanding (in shares) 58.5 58.5
Treasury stock, shares (in shares) 3.4 1.8