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Retirement Benefits (Summary of Benefit Plan Costs Incurred) (Details) - USD ($) $ in Millions | 3 Months Ended | 6 Months Ended
--- | --- | ---
Jun. 30, 2024 | Jun. 30, 2023 | Jun. 30, 2024 | Jun. 30, 2023
Pension Plan |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Net periodic benefit cost | $ (19) | $ (33) | $ (42) | $ (67)
Pension Plan | Union Electric Company |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Net periodic benefit cost | (10) | (17) | (22) | (35)
Pension Plan | Ameren Illinois Company |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Net periodic benefit cost | (8) | (13) | (17) | (27)
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) | (3) | (3) | (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) | (21) | (40) | (42)
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) | (7) | (14) | (15)
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) | (14) | (26) | (27)
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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