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Pension and Other Postretirement Benefits - Schedule of Expected Benefit Payments (Detail)
$ in Millions
Dec. 31, 2018
USD ($)
Pension  
Defined Benefit Plan Disclosure [Line Items]  
2019 $ 54.0
2020 55.0
2021 56.0
2022 58.0
2023 59.0
2024-2028 319.0
SERP  
Defined Benefit Plan Disclosure [Line Items]  
2019 3.0
2020 2.9
2021 2.9
2022 2.9
2023 2.8
2024-2028 13.6
PBOP  
Defined Benefit Plan Disclosure [Line Items]  
2019 4.5
2020 4.6
2021 4.6
2022 4.6
2023 4.5
2024-2028 $ 20.9