Postretirement Benefit Plans - Additional Information (Detail) $ in Millions	1 Months Ended	12 Months Ended
	Dec. 31, 2020 USD ($) retiree	Dec. 31, 2020 USD ($) retiree	Dec. 31, 2019 USD ($) retiree	Dec. 31, 2018 USD ($)
Defined Benefit Plan Disclosure [Line Items]
Assets set aside expected to be used to pay obligations of defined benefit pension plans and retiree medical and life insurance plans	$ 40,566	$ 40,566	$ 37,331
Aggregate liability for postemployment plans	42	42	42
Expected contribution, next year	1,000	$ 1,000
Hedge funds | Minimum
Defined Benefit Plan Disclosure [Line Items]
Fair value, investments, entities that calculate net asset value per share, investment redemption, notice period		1 month
Commingled equity funds
Defined Benefit Plan Disclosure [Line Items]
Fair value, investments, entities that calculate net asset value per share, investment redemption, notice period		90 days
Private equity funds | Minimum
Defined Benefit Plan Disclosure [Line Items]
Fair value, investments, entities that calculate net asset value per share, investment redemption, notice period		8 years
Private equity funds | Maximum
Defined Benefit Plan Disclosure [Line Items]
Fair value, investments, entities that calculate net asset value per share, investment redemption, notice period		12 years
Real estate funds | Minimum
Defined Benefit Plan Disclosure [Line Items]
Fair value, investments, entities that calculate net asset value per share, investment redemption, notice period		8 years
Real estate funds | Maximum
Defined Benefit Plan Disclosure [Line Items]
Fair value, investments, entities that calculate net asset value per share, investment redemption, notice period		10 years
Nonqualified Plan
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plan, liabilities	1,400	$ 1,400	1,400
Assets set aside expected to be used to pay obligations of defined benefit pension plans and retiree medical and life insurance plans	877	877	657
Unrecognized net actuarial losses	718	718	641
Unrecognized prior service credit		21	34
Benefit plan expense		59	108	$ 123
Qualified Defined  Benefit Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligation for defined benefit pension plans	51,300	51,300	48,600
Assets set aside expected to be used to pay obligations of defined benefit pension plans and retiree medical and life insurance plans	38,481	38,481	35,442	32,002
Unrecognized net actuarial losses	(21,040)	(21,040)	(20,609)
Increase (decrease) in projected benefit obligations due to a change in discount rate		4,900	5,800
Changes in longevity assumptions		426	(860)
Actual return on plan assets		$ 5,594	6,667
Actual return on plan assets (percent)		16.50%
Expected return on plan assets		$ 2,264	$ 2,300	$ 2,395
Expected long-term rate of return on assets		7.00%	7.00%	7.00%
Company contributions		$ 1,025	$ 1,000
Irrevocable Group Annuity Contract
Defined Benefit Plan Disclosure [Line Items]
Cash payment for contract	$ 1,400		$ 1,900
Irrevocable Group Annuity Contract | Qualified Defined  Benefit Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Number of retirees | retiree	13,500		20,000
Reimbursement Of Future Benefit Payments Plan
Defined Benefit Plan Disclosure [Line Items]
Cash payment for contract		$ 793
Reimbursement Of Future Benefit Payments Plan | Qualified Defined  Benefit Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Number of retirees | retiree		2,500