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Retirement Plans - Amounts in Accumulated Other Comprehensive Loss (Detail) - USD ($)
$ in Millions
Dec. 31, 2017
Dec. 31, 2016
Pension Benefits    
Defined Benefit Plan Disclosure [Line Items]    
Net actuarial (loss) gain $ (286.7) $ (297.4)
Net prior service credit 0.0 0.0
Total (286.7) (297.4)
Other Postretirement Benefits    
Defined Benefit Plan Disclosure [Line Items]    
Net actuarial (loss) gain 33.6 19.2
Net prior service credit 30.4 32.9
Total $ 64.0 $ 52.1