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
	Mar. 31, 2021 USD ($) Employee	Mar. 31, 2020 USD ($)	Jan. 01, 2021	Dec. 31, 2020
Defined Benefit Plan Disclosure [Line Items]
Curtailments		$ (3)
Action# 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Number of affected plan participants | Employee	840
Weighted average discount rate			3.06%	2.45%
Plan remeasurement date	Mar. 31, 2021
Decrease to accrued other postretirement benefits liability	$ (112)
Curtailments	(17)
Settlements	$ 26