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, 2018	Dec. 31, 2017
Defined Benefit Plan Disclosure [Line Items]
Accrued postretirement benefit liabilities	$ (11,410)	$ (15,703)
Accumulated other comprehensive loss (pre-tax) related to:
Accumulated other comprehensive loss, after tax	14,300	12,600
Qualified Defined Benefit Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Prepaid pension asset	107	112
Accrued postretirement benefit liabilities	(11,410)	(15,703)
Accumulated other comprehensive loss (pre-tax) related to:
Net actuarial losses	19,117	20,169
Prior service (credit) cost	(1,931)	(2,263)
Total	17,186	17,906
Accumulated other comprehensive loss, after tax	13,500	11,800
Adjustments, before tax	17,186	17,906
Retiree Medical and Life Insurance Plans
Defined Benefit Plan Disclosure [Line Items]
Prepaid pension asset	0	0
Accrued postretirement benefit liabilities	(704)	(719)
Accumulated other comprehensive loss (pre-tax) related to:
Net actuarial losses	236	331
Prior service (credit) cost	167	81
Total	403	412
Accumulated other comprehensive loss, after tax	316	252
Adjustments, before tax	403	412
Other Pension Plan
Accumulated other comprehensive loss (pre-tax) related to:
Total	542	705
Accumulated other comprehensive loss, after tax	428	479
Adjustments, before tax	$ 542	$ 705

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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