Postretirement Benefit Plans - Additional Information (Detail) retiree in Thousands, $ in Millions		1 Months Ended	12 Months Ended
	Jan. 01, 2020 USD ($)	Dec. 31, 2018 USD ($) retiree	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 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		$ 33,646	$ 33,646	$ 34,978
Aggregate liability for postemployment plans		46	46	60
Actuarial losses and net prior service cost related to postretirement benefit plans expected to be recognized in net periodic benefit cost during 2017		1,200	1,200
Actuarial losses and net prior service cost related to postretirement benefit plans expected to be recognized in net periodic benefit cost during 2017, net of tax			908
Expected pension plan expenses during next fiscal year		1,100	1,100
Nonqualified Plan
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plan, liabilities		1,200	1,200	1,300
Assets set aside expected to be used to pay obligations of defined benefit pension plans and retiree medical and life insurance plans		425	425	530
Unrecognized net actuarial losses		505	505	646
Unrecognized prior service credit			48	61
Benefit plan expense			123	126	$ 125
Qualified Defined Benefit Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligation for defined benefit pension plans		43,300	43,300	48,500
Accumulated benefit obligation in excess of plan assets		43,300	43,300	48,500
Assets set aside expected to be used to pay obligations of defined benefit pension plans and retiree medical and life insurance plans		32,002	32,002	33,095	$ 31,417
Unrecognized net actuarial losses		19,117	19,117	20,169
Actuarial losses related to postretirement benefit plans expected to be recognized in net periodic benefit cost during 2017		1,100	1,100
Actuarial losses related to postretirement benefit plans expected to be recognized in net periodic benefit cost during 2017, net of tax			841
Increase (decrease) in projected benefit obligations due to a change in discount rate			$ 3,500	$ 2,900
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
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
Buy-In Contract
Defined Benefit Plan Disclosure [Line Items]
Cash payment for contract			$ 810
Difference between insurance payment and pension obligations funded recognized in other comprehensive loss		$ 40
Buy-In Contract | Qualified Defined Benefit Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Reimbursable future benefit payments			770
Number of retirees | retiree		9
Buy-In Contract | Forecast
Defined Benefit Plan Disclosure [Line Items]
Loss related to litigation settlement	$ 350
Irrevocable Group Annuity Contract
Defined Benefit Plan Disclosure [Line Items]
Cash payment for contract			1,820
Difference between insurance payment and pension obligations funded recognized in other comprehensive loss		$ 60
Irrevocable Group Annuity Contract | Qualified Defined Benefit Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Reimbursable future benefit payments			$ 1,760
Number of retirees | retiree		32