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Pension Plans, Other Postretirement Benefits, and Defined-Contribution Plans - Estimated Benefit Payments Table (Detail)
$ in Millions
Dec. 31, 2017
USD ($)
Pension Plans, Defined Benefit [Member]  
Defined Benefit Plan Disclosure [Line Items]  
2018 $ 138
2019 155
2020 154
2021 166
2022 195
2023-2027 938
Other Postretirement Benefits Plan [Member]  
Defined Benefit Plan Disclosure [Line Items]  
2018 22
2019 21
2020 21
2021 20
2022 20
2023-2027 $ 90