Pension and Other Postretirement Benefits - Components of Net Periodic Benefit Cost (Parenthetical) (Detail) - USD ($) $ in Millions		12 Months Ended
	Dec. 31, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Expense related to pension and other postretirement benefits		$ 725
Settlements and lump sum benefits	$ (13)
Action Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Settlements and lump sum benefits	$ (341)
United States [Member]
Defined Benefit Plan Disclosure [Line Items]
Expense related to pension and other postretirement benefits		620
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost		561	$ 119	$ 83
Pension Benefits [Member] | United States [Member]
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost		358	74	21
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost		(32)	50	21
Expense related to pension and other postretirement benefits		$ 8	$ 8	$ 6