Employee Benefits - Additional Information (Detail) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Defined Benefit Plan Disclosure [Line Items]
Amount paid in severance benefits over the period	$ 100,000,000		$ 400,000,000
Severance charges	0	$ 0	339,000,000	$ 195,000,000
Post employment benefits liability	500,000,000		500,000,000
Pension
Defined Benefit Plan Disclosure [Line Items]
Remeasurement gain	454,000,000	0	454,000,000	0
Credit due to change in discount rate assumption	1,100,000,000		1,100,000,000
Charge due to difference between estimated and actual return on assets	600,000,000		600,000,000
Impact of plan change	200,000,000
Pension | Qualified Plan
Defined Benefit Plan Disclosure [Line Items]
Employer discretionary contribution amount			1,000,000,000	3,400,000,000
Pension | Nonqualified Plan
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan contributions by employer	0		0
Health Care and Life
Defined Benefit Plan Disclosure [Line Items]
Employer discretionary contribution amount			700,000,000
Remeasurement gain	$ 0	$ 0	$ 0	$ 0