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Retirement Benefits (Summary of Benefit Plan Costs Incurred) (Details) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2025 | Sep. 30, 2024 | Sep. 30, 2025 | Sep. 30, 2024
Pension Plan |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Net periodic benefit cost | $ (6) | $ (21) | $ (16) | $ (63)
Pension Plan | Union Electric Company |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Net periodic benefit cost | (4) | (11) | (11) | (33)
Pension Plan | Ameren Illinois Company |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Net periodic benefit cost | (1) | (8) | (3) | (25)
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 | (1) | (2) | (2) | (5)
Other Postretirement Benefit Plan, Defined Benefit |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Net periodic benefit cost | (20) | (19) | (60) | (59)
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 | (7) | (6) | (21) | (20)
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) | (39)
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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