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Fair Value Measurements (Schedule Of Carrying Amounts And Estimated Fair Values Of Financial Assets and Liabilities) (Details) - USD ($) $ in Millions | Sep. 30, 2025 | Dec. 31, 2024
--- | --- | ---
Carrying Amount(a) |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Long-Term Debt, Carrying Amount | $ 19,201 | $ 17,579
Debt Issuance Costs, Net | 139 | 129
Carrying Amount(a) | Union Electric Company |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Long-Term Debt, Carrying Amount | 8,246 | 7,745
Debt Issuance Costs, Net | 63 | 62
Carrying Amount(a) | Ameren Illinois Company |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Long-Term Debt, Carrying Amount | 6,258 | 5,852
Debt Issuance Costs, Net | 56 | 51
Fair Value |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Long-term Debt, Fair Value | 18,088 | 15,933
Fair Value | Level 2 |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Long-term Debt, Fair Value | 17,532 | 15,395
Fair Value | Level 3 |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Long-term Debt, Fair Value | 556 | 538
Fair Value | Union Electric Company |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Long-term Debt, Fair Value | 7,662 | 6,926
Fair Value | Union Electric Company | Level 2 |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Long-term Debt, Fair Value | 7,662 | 6,926
Fair Value | Union Electric Company | Level 3 |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Long-term Debt, Fair Value | 0 | 0
Fair Value | Ameren Illinois Company |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Long-term Debt, Fair Value | 5,815 | 5,243
Fair Value | Ameren Illinois Company | Level 2 |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Long-term Debt, Fair Value | 5,815 | 5,243
Fair Value | Ameren Illinois Company | Level 3 |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Long-term Debt, Fair Value | $ 0 | $ 0

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