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Employee Benefit Plans (Benefit Payments Expected Future Service) (Detail)
$ in Millions
Dec. 31, 2020
USD ($)
Pension Benefits  
Defined Benefit Plan Disclosure [Line Items]  
2021 $ 486
2022 512
2023 515
2024 533
2025 535
2026-2030 2,792
Other Postretirement Benefits  
Defined Benefit Plan Disclosure [Line Items]  
2021 110
2022 108
2023 106
2024 104
2025 101
2026-2030 $ 478