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Benefit Plans - Effect of One-Percentage-Point Change in Assumed Healthcare Cost Trend Rates (Detail)
$ in Millions
12 Months Ended
Dec. 31, 2017
USD ($)
Defined Benefit Plan Disclosure [Line Items]  
Effect on total annual service and interest cost $ 0.1
Effect on other post-retirement benefit obligations 0.7
Effect on total annual service and interest cost (0.1)
Effect on other post-retirement benefit obligations $ (0.6)