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Employee Benefit Plans - Components of Net Periodic Benefit Cost (Detail) - USD ($)
$ in Millions
 3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Defined Benefit Pension Plans    
Defined Benefit Plan Disclosure [Line Items]    
Service cost $ 3.0 $ 2.7
Interest cost 3.6 3.3
Expected return on plan assets (3.4) (2.5)
Amortization of prior service cost (credit) 0.1 0.1
Recognized actuarial losses 3.5 3.2
Total 6.8 6.8
OPEB    
Defined Benefit Plan Disclosure [Line Items]    
Interest cost 0.1 0.1
Amortization of prior service cost (credit) (0.2) $ (0.1)
Recognized actuarial losses $ 0.1