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Employee Benefit Plans - Components of Net Periodic Benefit Cost (Detail) - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended
Sep. 30, 2016
Sep. 30, 2015
Sep. 30, 2016
Sep. 30, 2015
Defined Benefit Pension Plans        
Defined Benefit Plan Disclosure [Line Items]        
Service cost $ 2.5 $ 2.8 $ 7.5 $ 8.4
Interest cost 3.9 3.8 11.6 11.6
Expected return on plan assets (3.9) (4.4) (11.6) (13.4)
Amortization of prior service cost 0.1 0.2 0.2 0.6
Recognized actuarial losses 2.9 3.4 8.9 10.3
Total 5.5 5.8 16.6 17.5
OPEB        
Defined Benefit Plan Disclosure [Line Items]        
Service cost     0.1 0.1
Interest cost 0.1 0.1 0.2 0.2
Amortization of prior service cost (0.2) (0.2) (0.6) (0.6)
Recognized actuarial losses $ 0.1 $ 0.1 0.2 0.2
Total     $ (0.1) $ (0.1)