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Pension and Other Postretirement Benefits - Schedule of Expected Benefit Payments (Detail)
$ in Millions
Dec. 31, 2017
USD ($)
Pension [Member]  
Defined Benefit Plan Disclosure [Line Items]  
2018 $ 51.0
2019 52.2
2020 53.6
2021 55.1
2022 56.6
2023-2027 308.3
SERP [Member]  
Defined Benefit Plan Disclosure [Line Items]  
2018 3.0
2019 3.0
2020 3.0
2021 2.9
2022 2.9
2023-2027 14.1
PBOP [Member]  
Defined Benefit Plan Disclosure [Line Items]  
2018 4.1
2019 4.2
2020 4.3
2021 4.3
2022 4.2
2023-2027 $ 19.6