Postretirement Benefit Plans - Amounts Recognized on Balance Sheets Related to Qualified Defined Benefit Pension Plans and Retiree Medical and Life Insurance Plans (Detail) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Accrued postretirement benefit liabilities	$ (13,234)	$ (11,410)
Accumulated other comprehensive loss (pre-tax) related to:
Accumulated other comprehensive loss, after tax	15,500	14,300
Qualified Defined Benefit Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Prepaid pension asset	2	107
Accrued postretirement benefit liabilities	(13,234)	(11,410)
Accumulated other comprehensive loss (pre-tax) related to:
Net actuarial losses	20,609	19,117
Prior service (credit) cost	(1,586)	(1,931)
Total	19,023	17,186
Accumulated other comprehensive loss, after tax	15,000	13,500
Adjustments, before tax	19,023	17,186
Retiree Medical and Life Insurance Plans
Defined Benefit Plan Disclosure [Line Items]
Prepaid pension asset	0	0
Accrued postretirement benefit liabilities	(337)	(704)
Accumulated other comprehensive loss (pre-tax) related to:
Net actuarial losses	(69)	236
Prior service (credit) cost	120	167
Total	51	403
Accumulated other comprehensive loss, after tax	39	316
Adjustments, before tax	51	403
Other Pension Plan
Accumulated other comprehensive loss (pre-tax) related to:
Total	667	542
Accumulated other comprehensive loss, after tax	527	428
Adjustments, before tax	$ 667	$ 542

Amount, after tax, of accumulated other comprehensive (income) loss for defined benefit plan, that has not been recognized in net periodic benefit cost (credit).

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Amount of liability, recognized in statement of financial position, for defined benefit pension plan, classified as noncurrent. Excludes other postretirement benefit plan.

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Amount, before tax, of accumulated other comprehensive (income) loss for defined benefit plan, that has not been recognized in net periodic benefit cost (credit).

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Amount, before tax, of accumulated other comprehensive income (loss) for gain (loss) of defined benefit plan, that has not been recognized in net periodic benefit (cost) credit.

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Amount, before tax, of accumulated other comprehensive (income) loss for cost (credit) of benefit change attributable to participants' prior service from plan amendment or plan initiation of defined benefit plan, that has not been recognized in net periodic benefit cost (credit).

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Amount of asset, recognized in statement of financial position, for overfunded defined benefit pension and other postretirement plans.

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