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	6 Months Ended
	Jun. 30, 2019 USD ($)	Jun. 30, 2020 USD ($) Participant	Jun. 30, 2019 USD ($)	Apr. 30, 2020	Jan. 31, 2020	Dec. 31, 2019
Defined Benefit Plan Disclosure [Line Items]
Increase to accrued pension benefits liability		$ 202
Curtailments	$ 38	$ 2	$ 38
Action# 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Number of affected plan participants | Participant		20
Weighted average discount rate					2.75%	3.15%
Plan remeasurement date		Jan. 31, 2020
Increase to accrued pension benefits liability		$ 18
Curtailments		$ 1
Action# 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Number of affected plan participants | Participant		430
Weighted average discount rate					2.75%	3.20%
Plan remeasurement date		Jan. 31, 2020
Increase to accrued pension benefits liability		$ 28
Curtailments		$ 2
Action# 3a [Member]
Defined Benefit Plan Disclosure [Line Items]
Number of affected plan participants | Participant		300
Weighted average discount rate				2.92%		3.25%
Plan remeasurement date		Apr. 30, 2020
Increase to accrued pension benefits liability		$ 156
Curtailments		$ 1
Action# 3b [Member]
Defined Benefit Plan Disclosure [Line Items]
Number of affected plan participants | Participant		600
Weighted average discount rate				3.44%		3.75%
Plan remeasurement date		Apr. 30, 2020
Curtailments		$ (2)