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, 2017 USD ($)
Pension benefits
Defined Benefit Plan Disclosure [Line Items]
2018	$ 435
2019	440
2020	440
2021	445
2022	450
Thereafter	2,265
Total benefit payments	4,475
Gross Other post- retirement benefits
Defined Benefit Plan Disclosure [Line Items]
2018	90
2019	90
2020	90
2021	90
2022	90
Thereafter	335
Total benefit payments	785
Medicare Part D subsidy receipts
Defined Benefit Plan Disclosure [Line Items]
2018	5
2019	5
2020	5
2021	5
2022	5
Thereafter	25
Total benefit payments	50
Net Other post- retirement benefits
Defined Benefit Plan Disclosure [Line Items]
2018	85
2019	85
2020	85
2021	85
2022	85
Thereafter	310
Total benefit payments	$ 735