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Estimated Future Benefit Payments (Detail) $ in Millions | Dec. 31, 2017 USD ($)
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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

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