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Retirement Plans and Other Post-Retirement Benefits - Components of Amounts Recognized as Accumulated Other Comprehensive Income on Pre-Tax Basis (Detail) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Pension Benefits [Member]    
Defined Benefit Plan Disclosure [Line Items]    
Prior service cost/(credit) $ 12.0 $ 15.1
Net actuarial loss 185.0 181.3
Other Benefits [Member]    
Defined Benefit Plan Disclosure [Line Items]    
Prior service cost/(credit) (0.8) (1.1)
Net actuarial loss $ (4.8) $ 5.5