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Employee Benefits - Expected Future Benefit Payments (Detail)
$ in Millions
Dec. 31, 2017
USD ($)
Pension Plans [Member]  
Defined Benefit Plan Disclosure [Line Items]  
2018 $ 201
2019 215
2020 232
2021 250
2022 260
2023 - 2027 1,564
Postretirement Welfare Plan [Member]  
Defined Benefit Plan Disclosure [Line Items]  
2018 10
2019 9
2020 9
2021 8
2022 8
2023 - 2027 29
Medicare Part D Subsidy [Member]  
Defined Benefit Plan Disclosure [Line Items]  
2018 2
2019 1
2020 1
2021 1
2022 1
2023 - 2027 $ 4