Pension and Other Postretirement Benefits - Summary of Information in Curtailment or Settlement of Benefits Requiring Remeasurement, Update to Discount Rates Used to Determine Benefit Obligations of Affected Plans (Detail) $ in Millions	3 Months Ended	9 Months Ended
	Sep. 30, 2018 USD ($)	Sep. 30, 2019 USD ($) Participant	Sep. 30, 2018 USD ($)	Jun. 30, 2019	May 31, 2019	Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Curtailment charge	$ (174)		$ (318)
Action# 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Number of affected plan participants | Participant		700
Weighted average discount rate					3.15%	3.85%
Plan remeasurement date		May 31, 2019
Increase to accrued pension benefits liability		$ 52
Curtailment charge		$ 38
Action# 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Number of affected plan participants | Participant		900
Weighted average discount rate				3.00%		3.80%
Plan remeasurement date		Jun. 30, 2019
Increase to accrued pension benefits liability		$ 23