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Retirement Benefits (Narrative) (Details) $ in Millions | 12 Months Ended
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Dec. 31, 2025 | Dec. 31, 2024 USD ($) bond | Dec. 31, 2023 USD ($) | Dec. 31, 2022
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Defined Benefit Plan, Funded (Unfunded) Status of Plan |  | $ 734 | $ 551 |
Defined Benefit Plan Assumptions Used Calculating Benefit Obligation, Change in Discount Rate |  | 0.45% |  |
Number of high-quality corporate bonds | bond |  | 860 |  |
Defined benefit plan estimated future employer contributions over next five years |  | $ 170 |  |
Collateralized loan percentage compared to asset's market value |  | 102.00% |  |
Securities Loaned |  | $ 454 | 234 |
Pension Benefits |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Defined Benefit Plan, Funded (Unfunded) Status of Plan |  | $ 48 | $ 14 |
Expected return on plan assets |  | 6.75% | 6.75% | 6.50%
Pension Benefits | Forecast |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Expected return on plan assets | 6.75% |  |  |
Ameren Missouri | Pension Benefits |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Future funding requirement, percentage |  | 40.00% |  |
Ameren Illinois | Pension Benefits |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Future funding requirement, percentage |  | 50.00% |  |

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- Definition The employer's best estimate, as soon as it can be reasonably determined, of the contributions expected to be paid to the plan during the next five fiscal years beginning after the date of the latest statement of financial position. Estimated contributions may be presented in the aggregate combining (1) contributions required by funding regulations or laws, (2) discretionary contributions, and (3) noncash contributions. + References No definition available. + Details
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