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	1 Months Ended	12 Months Ended
	Sep. 30, 2018 USD ($) Participant	Dec. 31, 2018 USD ($) Participant Plan	Jan. 17, 2018 Employee	Dec. 31, 2017
Defined Benefit Plan Disclosure [Line Items]
Number of affected plan participants | Participant		20,600
(Decrease) increase to accrued pension benefits liability		$ (143)
Decrease to accrued other postretirement benefits liability		(93)
Curtailment charge (gain)		(23)
Settlement charge		$ 341
Action# 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Number of plans | Plan		3
Number of affected plan participants		800	800
Weighted average discount rate		3.80%		3.65%
Plan remeasurement date		Jan. 31, 2018
(Decrease) increase to accrued pension benefits liability		$ (57)
Curtailment charge (gain)		$ 5
Action# 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Number of plans | Plan		1
Number of affected plan participants | Participant		700
Weighted average discount rate		3.43%		3.29%
Plan remeasurement date		Jan. 31, 2018
Decrease to accrued other postretirement benefits liability		$ (7)
Curtailment charge (gain)		$ (28)
Action# 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Number of plans | Plan		2
Number of affected plan participants | Participant		2,100
Weighted average discount rate		3.60%		3.43%
Plan remeasurement date		Mar. 31, 2018
(Decrease) increase to accrued pension benefits liability		$ 24
Settlement charge		$ 167
Action# 4 [Member]
Defined Benefit Plan Disclosure [Line Items]
Number of plans | Plan		3
Number of affected plan participants | Participant		11,500
Weighted average discount rate		4.39%		3.70%
Plan remeasurement date		Jul. 31, 2018
(Decrease) increase to accrued pension benefits liability		$ (110)
Settlement charge		$ 230
Action# 5 [Member]
Defined Benefit Plan Disclosure [Line Items]
Number of plans | Plan		1
Number of affected plan participants | Participant	5,500	5,500
Weighted average discount rate		4.35%		3.61%
Plan remeasurement date		Jul. 31, 2018
Decrease to accrued other postretirement benefits liability		$ (86)
Settlement charge	$ 23	$ (56)