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Defined Benefit Plans - Amounts Recognized in Consolidated Balance Sheets (Detail) - USD ($)
$ in Millions
Dec. 31, 2017
Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]    
Accrued benefit liability $ (175.9) $ (216.2)
Pension Benefits [Member]    
Defined Benefit Plan Disclosure [Line Items]    
Current benefit payment liability (1.1) (1.0)
Accrued benefit liability (174.7) (213.0)
Net amount recognized (175.8) (214.0)
Postretirement Benefits [Member]    
Defined Benefit Plan Disclosure [Line Items]    
Current benefit payment liability (0.2) (0.4)
Accrued benefit liability (1.4) (3.2)
Net amount recognized $ (1.6) $ (3.6)