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Postretirement Benefit Plans - Estimated Future Benefit Payments (Detail)
$ in Millions
Dec. 31, 2018
USD ($)
Qualified Defined Benefit Pension Plans  
Defined Benefit Plan Disclosure [Line Items]  
Contributions expected to made by employer in 2018 $ 5,000
Expected Future Benefit Payment [Abstract]  
Payments in 2019 2,350
Payments in 2020 2,390
Payments in 2021 2,470
Payments in 2022 2,550
Payments in 2023 2,610
Payments in 2024 - 2028 13,670
Retiree Medical and Life Insurance Plans  
Expected Future Benefit Payment [Abstract]  
Payments in 2019 170
Payments in 2020 180
Payments in 2021 180
Payments in 2022 180
Payments in 2023 170
Payments in 2024 - 2028 $ 810