Pension and Other Postretirement Benefits - Schedule of Benefit Payments Expected to be Paid and Expected Medicare Part D Subsidy Receipts (Detail) $ in Millions	Dec. 31, 2018 USD ($)
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
2019	$ 420
2020	420
2021	425
2022	425
2023	425
2024 through 2028	2,060
Total benefit payments	4,175
Gross Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
2019	110
2020	110
2021	110
2022	105
2023	105
2024 through 2028	340
Total benefit payments	880
Medicare Part D Subsidy Receipts [Member]
Defined Benefit Plan Disclosure [Line Items]
2019	10
2020	10
2021	5
2022	5
2023	5
2024 through 2028	25
Total benefit payments	60
Net Other Post Retirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
2019	100
2020	100
2021	105
2022	100
2023	100
2024 through 2028	315
Total benefit payments	$ 820