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Related Party Transactions (Narrative) (Details) - USD ($) $ in Millions | Mar. 31, 2025 | Dec. 31, 2024
--- | --- | ---
Related Party Transaction [Line Items] |  |
Other assets | $ 1,143 | $ 1,123
Union Electric Company |  |
Related Party Transaction [Line Items] |  |
Other assets | 239 | 254
Ameren Illinois Company |  |
Related Party Transaction [Line Items] |  |
Other assets | 731 | 697
Ameren Services Support Services Agreement | Union Electric Company | Related Party |  |
Related Party Transaction [Line Items] |  |
Other assets | 27 | 29
Ameren Services Support Services Agreement | Ameren Illinois Company | Related Party |  |
Related Party Transaction [Line Items] |  |
Other assets | $ 30 | $ 32

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