Related Party Transactions (Narrative) (Details) - USD ($) $ in Millions	Sep. 30, 2026	Dec. 31, 2025
Related Party Transaction [Line Items]
Other assets	$ 1,396	$ 1,154
Union Electric Company
Related Party Transaction [Line Items]
Other assets	352	244
Ameren Illinois Company
Related Party Transaction [Line Items]
Other assets	885	789
Ameren Services Support Services Agreement \| Union Electric Company \| Related Party
Related Party Transaction [Line Items]
Other assets	32	36
Ameren Services Support Services Agreement \| Ameren Illinois Company \| Related Party
Related Party Transaction [Line Items]
Other assets	$ 34	$ 38

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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+ Details