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Related Party Transactions (Narrative) (Details) - USD ($) $ in Millions	Sep. 30, 2025	Dec. 31, 2024
Related Party Transaction [Line Items]
Other assets	$ 1,130	$ 1,123
Union Electric Company
Related Party Transaction [Line Items]
Other assets	235	254
Ameren Illinois Company
Related Party Transaction [Line Items]
Other assets	756	697
Ameren Services Support Services Agreement | Union Electric Company | Related Party
Related Party Transaction [Line Items]
Other assets	18	29
Ameren Services Support Services Agreement | Ameren Illinois Company | Related Party
Related Party Transaction [Line Items]
Other assets	$ 20	$ 32

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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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