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Employee Benefit Plans (Estimated Cash Flows) (Details)
$ in Millions
Dec. 31, 2020
USD ($)
Pension Plans | Qualified Plan  
Defined Benefit Plan Disclosure [Line Items]  
Estimated future benefit payments during 2021 $ 292
Estimated future benefit payments during 2022 304
Estimated future benefit payments during 2023 317
Estimated future benefit payments during 2024 306
Estimated future benefit payments during 2025 303
Estimated future benefit payments during 2026-2030 1,476
Pension Plans | Nonqualified Plan  
Defined Benefit Plan Disclosure [Line Items]  
Estimated 2021 employer contributions 25
Estimated future benefit payments during 2021 25
Estimated future benefit payments during 2022 22
Estimated future benefit payments during 2023 22
Estimated future benefit payments during 2024 21
Estimated future benefit payments during 2025 20
Estimated future benefit payments during 2026-2030 89
Postretirement Benefits  
Defined Benefit Plan Disclosure [Line Items]  
Estimated 2021 employer contributions 24
Estimated future benefit payments during 2021 24
Estimated future benefit payments during 2022 23
Estimated future benefit payments during 2023 23
Estimated future benefit payments during 2024 23
Estimated future benefit payments during 2025 22
Estimated future benefit payments during 2026-2030 $ 104