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Related Party Transactions (Narrative) (Details) - USD ($) $ in Millions | Sep. 30, 2023 | Dec. 31, 2022
--- | --- | ---
Related Party Transaction [Line Items] |  |
Other assets | $ 882 | $ 768
Union Electric Company |  |
Related Party Transaction [Line Items] |  |
Other assets | 137 | 140
Ameren Illinois Company |  |
Related Party Transaction [Line Items] |  |
Other assets | 565 | 482
Ameren Services Support Services Agreement | Union Electric Company | Related Party |  |
Related Party Transaction [Line Items] |  |
Other assets | 28 | 41
Ameren Services Support Services Agreement | Ameren Illinois Company | Related Party |  |
Related Party Transaction [Line Items] |  |
Other assets | $ 31 | $ 43

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