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Benefit Plans - Estimated Future Minimum Benefit Payments (Detail) - USD ($)
$ in Millions
Dec. 31, 2018
Dec. 31, 2017
Pension Benefits [Member]    
Defined Benefit Plan Disclosure [Line Items]    
Expected future benefit payments 2019 $ 218  
Expected future benefit payments 2020 223  
Expected future benefit payments 2021 210  
Expected future benefit payments 2022 218  
Expected future benefit payments 2023 214  
Expected future benefit payments 2024 - 2028 $ 1,026  
Other Postretirement Benefits [Member]    
Defined Benefit Plan Disclosure [Line Items]    
Expected future benefit payments 2019   $ 5
Expected future benefit payments 2020   5
Expected future benefit payments 2021   5
Expected future benefit payments 2022   4
Expected future benefit payments 2023   4
Expected future benefit payments 2024 - 2028   $ 16