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Postretirement Benefits Other Than Pensions - Estimated Post retirement Health Care Plan Benefit Payments (Detail) - Postretirement Benefits
$ in Millions
Dec. 31, 2017
USD ($)
Defined Benefit Plan Disclosure [Line Items]  
2018 $ 2.2
2019 2.3
2020 2.4
2021 2.3
2022 2.4
Next five years $ 11.3