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Related Party Transactions (Narrative) (Details) - USD ($) $ in Millions | Mar. 31, 2024 | Dec. 31, 2023
--- | --- | ---
Related Party Transaction [Line Items] |  |
Other assets | $ 964 | $ 921
Union Electric Company |  |
Related Party Transaction [Line Items] |  |
Other assets | 157 | 152
Ameren Illinois Company |  |
Related Party Transaction [Line Items] |  |
Other assets | 632 | 603
Ameren Services Support Services Agreement | Union Electric Company | Related Party |  |
Related Party Transaction [Line Items] |  |
Other assets | 28 | 31
Ameren Services Support Services Agreement | Ameren Illinois Company | Related Party |  |
Related Party Transaction [Line Items] |  |
Other assets | $ 31 | $ 34

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