Short-Term Debt and Liquidity (Short-Term Debt outstanding) (Details) - USD ($) $ in Millions	Sep. 30, 2026	Dec. 31, 2025
Short-term Debt [Line Items]
Short-term debt	$ 496	$ 643
Union Electric Company
Short-term Debt [Line Items]
Short-term debt	100	471
Ameren Illinois Company
Short-term Debt [Line Items]
Short-term debt	188	17
Commercial Paper
Short-term Debt [Line Items]
Short-term debt	496	643
Commercial Paper \| Ameren (parent)
Short-term Debt [Line Items]
Short-term debt	208	155
Commercial Paper \| Union Electric Company
Short-term Debt [Line Items]
Short-term debt	100	471
Commercial Paper \| Ameren Illinois Company
Short-term Debt [Line Items]
Short-term debt	$ 188	$ 17

Reflects the total carrying amount as of the balance sheet date of debt having initial terms less than one year or the normal operating cycle, if longer.

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