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Reconciliation of Tax Loss Carryovers, Credit Carryovers and Valuation Allowances (Detail) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Income Taxes      
Net Operating Losses and Tax Credits, Beginning balance $ 18.2 $ 14.6  
Net Operating Losses and Tax Credits, Additions 1.6 3.7  
Net Operating Losses and Tax Credits, Reductions   (0.1)  
Net Operating Losses and Tax Credits, Ending balance 19.8 18.2 $ 14.6
Balance at Beginning of Year 13.1    
Balance at End of Year 15.0 13.1  
Valuation Allowance For Deferred Tax Assets      
Income Taxes      
Balance at Beginning of Year 13.1 11.0  
Valuation Allowances, Additions 1.9 2.1  
Valuation Allowances, Reductions 0.0 0.0 (0.2)
Balance at End of Year $ 15.0 $ 13.1 $ 11.0