Postretirement Benefit Plans - Estimated Future Benefit Payments (Detail) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019
Qualified Defined Benefit Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Company contributions	$ 1,025	$ 1,000
Expected Future Benefit Payment [Abstract]
2021	2,290
2022	2,350
2023	2,440
2024	2,510
2025	2,570
2026 – 2030	13,190
Retiree Medical and Life Insurance Plans
Defined Benefit Plan Disclosure [Line Items]
Company contributions	45	$ 59
Expected Future Benefit Payment [Abstract]
2021	160
2022	150
2023	150
2024	150
2025	150
2026 – 2030	$ 660

Amount of contribution received by defined benefit plan from employer which increases plan assets.

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Amount of benefit for defined benefit plan expected to be paid in five fiscal years after fifth fiscal year following current fiscal year.

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