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Related Party Transactions (Narrative) (Details) - USD ($) $ in Millions | Sep. 30, 2024 | Dec. 31, 2023
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Related Party Transaction [Line Items] |  |
Other assets | $ 1,081 | $ 921
Union Electric Company |  |
Related Party Transaction [Line Items] |  |
Other assets | 204 | 152
Ameren Illinois Company |  |
Related Party Transaction [Line Items] |  |
Other assets | 679 | 603
Ameren Services Support Services Agreement | Union Electric Company | Related Party |  |
Related Party Transaction [Line Items] |  |
Other assets | 19 | 31
Ameren Services Support Services Agreement | Ameren Illinois Company | Related Party |  |
Related Party Transaction [Line Items] |  |
Other assets | $ 22 | $ 34

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