Postretirement Benefit Plans - Additional Information (Detail) retiree in Thousands, $ in Millions	1 Months Ended		12 Months Ended
	Dec. 31, 2019 USD ($) retiree	Dec. 31, 2018 USD ($) contract retiree	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($) retiree	Dec. 31, 2017 USD ($)
Defined Benefit Plan Disclosure [Line Items]
Number of insurance contracts purchased | contract		2
Number of retirees | retiree				32
Assets set aside expected to be used to pay obligations of defined benefit pension plans and retiree medical and life insurance plans	$ 37,331	$ 33,646	$ 37,331	$ 33,646
Aggregate liability for postemployment plans	42	46	42	46
Actuarial losses and net prior service cost related to postretirement benefit plans expected to be recognized in net periodic benefit cost during 2020	559		559
Actuarial losses and net prior service cost related to postretirement benefit plans expected to be recognized in net periodic benefit cost during 2020, net of tax			441
Expected pension plan expenses during next fiscal year	(115)		(115)
Nonqualified Plan
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plan, liabilities	1,400	1,200	1,400	1,200
Assets set aside expected to be used to pay obligations of defined benefit pension plans and retiree medical and life insurance plans	657	425	657	425
Unrecognized net actuarial losses	641	505	641	505
Unrecognized prior service credit			34	48
Benefit plan expense			108	123	$ 126
Qualified Defined Benefit Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligation for defined benefit pension plans	48,600	43,300	48,600	43,300
Assets set aside expected to be used to pay obligations of defined benefit pension plans and retiree medical and life insurance plans	35,442	32,002	35,442	32,002	33,095
Unrecognized net actuarial losses	20,609	19,117	20,609	19,117
Actuarial losses related to postretirement benefit plans expected to be recognized in net periodic benefit cost during 2020	507		507
Actuarial losses related to postretirement benefit plans expected to be recognized in net periodic benefit cost during 2020, net of tax			399
Increase (decrease) in projected benefit obligations due to a change in discount rate			5,800	3,500
Company contributions			1,000	5,000
Changes in longevity assumptions			860	(162)
Actual return on plan assets			$ 6,667	(1,893)
Actual return on plan assets (percent)			21.00%
Expected return on plan assets			$ 2,300	$ 2,395	$ 2,249
Expected long-term rate of return on assets			7.00%	7.00%	7.50%
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
Irrevocable Group Annuity Contract
Defined Benefit Plan Disclosure [Line Items]
Cash payment for contract	1,900	$ 2,600
Difference between insurance payment and pension obligations funded recognized in other comprehensive loss	$ 45
Irrevocable Group Annuity Contract | Qualified Defined Benefit Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Number of retirees | retiree	20	32
Reimbursable future benefit payments	$ 1,900
Buy-In Contract | Qualified Defined Benefit Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Number of retirees | retiree		9		9