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Retirement Benefits (Summary of Benefit Plan Costs Incurred) (Details) - USD ($) $ in Millions | 3 Months Ended
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Mar. 31, 2024 | Mar. 31, 2023
Pension Plan |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |
Net periodic benefit cost | $ (23) | $ (34)
Pension Plan | Union Electric Company |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |
Net periodic benefit cost | (12) | (18)
Pension Plan | Ameren Illinois Company |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |
Net periodic benefit cost | (9) | (14)
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) | (2)
Other Postretirement Benefit Plan, Defined Benefit |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |
Net periodic benefit cost | (20) | (21)
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) | (8)
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)
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

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