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Retirement Benefits (Summary of Benefit Plan Costs Incurred) (Details) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2024 | Sep. 30, 2023 | Sep. 30, 2024 | Sep. 30, 2023
Pension Plan |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Net periodic benefit cost | $ (21) | $ (48) | $ (63) | $ (115)
Pension Plan | Union Electric Company |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Net periodic benefit cost | (11) | (24) | (33) | (59)
Pension Plan | Ameren Illinois Company |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Net periodic benefit cost | (8) | (21) | (25) | (48)
Pension Plan | Other Affiliated Entities And Intercompany Eliminations |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Net periodic benefit cost | (2) | (3) | (5) | (8)
Other Postretirement Benefit Plan, Defined Benefit |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Net periodic benefit cost | (19) | (21) | (59) | (63)
Other Postretirement Benefit Plan, Defined Benefit | Union Electric Company |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Net periodic benefit cost | (6) | (8) | (20) | (23)
Other Postretirement Benefit Plan, Defined Benefit | Ameren Illinois Company |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Net periodic benefit cost | (13) | (13) | (39) | (40)
Other Postretirement Benefit Plan, Defined Benefit | Other Affiliated Entities And Intercompany Eliminations |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Net periodic benefit cost | $ 0 | $ 0 | $ 0 | $ 0

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