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, 2015 USD ($)
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
2016	$ 910
2017	900
2018	910
2019	910
2020	920
2021 through 2025	4,650
Total benefit payments	9,200
Gross Other Post-Retirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
2016	230
2017	225
2018	225
2019	220
2020	220
2021 through 2025	975
Total benefit payments	2,095
Medicare Part D Subsidy Receipts [Member]
Defined Benefit Plan Disclosure [Line Items]
2016	15
2017	15
2018	15
2019	15
2020	20
2021 through 2025	80
Total benefit payments	160
Net Other Post-Retirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
2016	215
2017	210
2018	210
2019	205
2020	200
2021 through 2025	895
Total benefit payments	$ 1,935