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, 2021 USD ($) Employee	Sep. 30, 2021 USD ($) Employee	Jun. 30, 2021	Mar. 31, 2021	Dec. 31, 2020
Action# 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Number of affected plan participants | Employee	840	840
Weighted average discount rate				3.06%	2.45%
Plan remeasurement date		Mar. 31, 2021
Decrease to accrued other postretirement benefits liability		$ (106)
Curtailment gain		(17)
Settlement charge		$ 26
Action# 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Number of affected plan participants | Employee	120	120
Weighted average discount rate			2.71%		2.38%
Plan remeasurement date		Jun. 30, 2021
Increase (decrease) to accrued other pension benefits liability		$ (90)
Settlement charge		$ 39
Action# 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Number of affected plan participants | Employee	20	20
Weighted average discount rate	2.74%	2.74%			2.71%
Plan remeasurement date		Sep. 30, 2021
Increase (decrease) to accrued other pension benefits liability	$ 7	$ 7
Settlement charge	$ 7	$ 7
Action# 4 [Member]
Defined Benefit Plan Disclosure [Line Items]
Number of affected plan participants | Employee	20	20
Weighted average discount rate	1.53%	1.53%			1.34%
Plan remeasurement date		Sep. 30, 2021
Increase (decrease) to accrued other pension benefits liability	$ (38)	$ (38)
Settlement charge	$ 1	$ 1