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Estimated Future Benefit Payments (Detail)
$ in Millions
Dec. 31, 2017
USD ($)
Defined Benefit Plan Disclosure [Line Items]  
2018 $ 6
2019 6
2020 6
2021 7
2022 7
Years 2023 - 2027 43
Pension Benefits  
Defined Benefit Plan Disclosure [Line Items]  
2018 305
2019 326
2020 331
2021 357
2022 369
Years 2023 - 2027 1,912
Other Postretirement Benefits  
Defined Benefit Plan Disclosure [Line Items]  
2018 113
2019 118
2020 121
2021 124
2022 126
Years 2023 - 2027 $ 646