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Defined Benefit Plans - Fair Value of Pension Assets by Major Category of Plan Assets (Detail) - USD ($)
$ in Millions
Dec. 31, 2017
Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]    
Fair value of pension assets $ 656.6 $ 577.7
Group annuity/insurance contracts (Level 3)    
Defined Benefit Plan Disclosure [Line Items]    
Fair value of pension assets 23.3 22.8
Cash and cash equivalents    
Defined Benefit Plan Disclosure [Line Items]    
Fair value of pension assets 12.5 6.9
Equity    
Defined Benefit Plan Disclosure [Line Items]    
Fair value of pension assets 285.9 258.8
Fixed income    
Defined Benefit Plan Disclosure [Line Items]    
Fair value of pension assets 277.7 235.4
Multi-strategy hedge funds    
Defined Benefit Plan Disclosure [Line Items]    
Fair value of pension assets 24.6 23.1
Real estate [Member]    
Defined Benefit Plan Disclosure [Line Items]    
Fair value of pension assets $ 32.6 $ 30.7