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Retirement Plans - Expected Benefit Payments (Detail)
$ in Millions
Dec. 31, 2017
USD ($)
Pension Benefits  
Defined Benefit Plan Disclosure [Line Items]  
2018 $ 46.8
2019 47.2
2020 48.6
2021 50.3
2022 51.2
2023-2027 268.2
Other Postretirement Benefits  
Defined Benefit Plan Disclosure [Line Items]  
2018 25.7
2019 25.4
2020 25.1
2021 24.8
2022 24.5
2023-2027 117.3
Estimated Subsidy Reimbursements  
Defined Benefit Plan Disclosure [Line Items]  
2018 1.3
2019 1.3
2020 1.2
2021 1.1
2022 1.1
2023-2027 $ 5.1