Postretirement Benefit Plans - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
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	$ 34,978	$ 33,204
Aggregate liability for postemployment plans	60	63
Actuarial losses and net prior service cost related to postretirement benefit plans expected to be recognized in net periodic benefit cost during 2017	1,500
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	1,200
Expected pension plan expenses during next fiscal year	$ 1,400
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
Minimum | Private equity funds
Defined Benefit Plan Disclosure [Line Items]
Fair value, investments, entities that calculate net asset value per share, investment redemption, notice period	8 years
Minimum | Real estate funds
Defined Benefit Plan Disclosure [Line Items]
Fair value, investments, entities that calculate net asset value per share, investment redemption, notice period	8 years
Minimum | Hedge funds
Defined Benefit Plan Disclosure [Line Items]
Fair value, investments, entities that calculate net asset value per share, investment redemption, notice period	1 month
Maximum | Private equity funds
Defined Benefit Plan Disclosure [Line Items]
Fair value, investments, entities that calculate net asset value per share, investment redemption, notice period	12 years
Maximum | Real estate funds
Defined Benefit Plan Disclosure [Line Items]
Fair value, investments, entities that calculate net asset value per share, investment redemption, notice period	10 years
Investments measured at NAV
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	$ 6,823	5,547
Investments measured at NAV | Private equity funds
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	4,334	3,614
Investments measured at NAV | Real estate funds
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	1,611	1,411
Investments measured at NAV | Hedge funds
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	711	462
Nonqualified Plan
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plan, liabilities	1,300	1,200
Assets set aside expected to be used to pay obligations of defined benefit pension plans and retiree medical and life insurance plans	530	460
Unrecognized net actuarial losses	646	642
Unrecognized prior service credit	61	74
Benefit plan expense	126	125	$ 117
Qualified Defined Benefit Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligation for defined benefit pension plans	48,500	44,900
Accumulated benefit obligation in excess of plan assets	48,500	44,800
Assets set aside expected to be used to pay obligations of defined benefit pension plans and retiree medical and life insurance plans	33,095	31,417	$ 32,096
Unrecognized net actuarial losses	20,169	20,184
Actuarial losses related to postretirement benefit plans expected to be recognized in net periodic benefit cost during 2017	1,400
Actuarial losses related to postretirement benefit plans expected to be recognized in net periodic benefit cost during 2017, net of tax	1,100
Increase (decrease) in projected benefit obligations due to a change in discount rate	$ 2,900	$ 1,400