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, 2020	Dec. 31, 2019
Defined Benefit Plan Disclosure [Line Items]
Accrued postretirement benefit liabilities	$ (12,874)	$ (13,234)
Accumulated other comprehensive loss (pre-tax) related to:
Accumulated other comprehensive loss, after tax	16,200	15,500
Qualified Defined Benefit Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Prepaid pension asset	3	2
Accrued postretirement benefit liabilities	(12,874)	(13,234)
Accumulated other comprehensive loss (pre-tax) related to:
Net actuarial losses	21,040	20,609
Prior service (credit) cost	(1,235)	(1,586)
Total	19,805	19,023
Accumulated other comprehensive loss, after tax	15,600	15,000
Adjustments, before tax	19,805	19,023
Retiree Medical and Life Insurance Plans
Defined Benefit Plan Disclosure [Line Items]
Prepaid pension asset	0	0
Accrued postretirement benefit liabilities	(186)	(337)
Accumulated other comprehensive loss (pre-tax) related to:
Net actuarial losses	(119)	(69)
Prior service (credit) cost	73	120
Total	(46)	51
Accumulated other comprehensive loss, after tax	(37)	39
Adjustments, before tax	(46)	51
Other Pension Plan
Accumulated other comprehensive loss (pre-tax) related to:
Total	782	667
Accumulated other comprehensive loss, after tax	617	527
Adjustments, before tax	$ 782	$ 667

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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