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Fair Value Measurements (Schedule Of Carrying Amounts And Estimated Fair Values Of Financial Assets and Liabilities) (Details) - USD ($) $ in Millions | Mar. 31, 2023 | Dec. 31, 2022 | Mar. 31, 2022 | Dec. 31, 2021
--- | --- | --- | --- | ---
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |
Cash, cash equivalents, and restricted cash | $ 237 | $ 216 | $ 154 | $ 155
Short-term debt | 1,248 | 1,070 |  |
Union Electric Company |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |
Cash, cash equivalents, and restricted cash | 11 | 13 | 9 | 8
Short-term debt | 197 | 329 |  |
Ameren Illinois Company |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |
Cash, cash equivalents, and restricted cash | 217 | 191 | $ 137 | $ 133
Short-term debt | 324 | 264 |  |
Carrying Amount |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |
Cash, cash equivalents, and restricted cash | 237 | 216 |  |
Investments in industrial development revenue bonds |  | 240 |  |
Short-term debt | 1,248 | 1,070 |  |
Long-term debt (including current portion) | 14,281 | 14,025 |  |
Debt Issuance Costs, Net | 103 | 99 |  |
Carrying Amount | Union Electric Company |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |
Cash, cash equivalents, and restricted cash | 11 | 13 |  |
Investments in industrial development revenue bonds |  | 240 |  |
Short-term debt | 197 | 329 |  |
Long-term debt (including current portion) | 6,341 | 6,086 |  |
Debt Issuance Costs, Net | 45 | 41 |  |
Carrying Amount | Ameren Illinois Company |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |
Cash, cash equivalents, and restricted cash | 217 | 191 |  |
Short-term debt | 324 | 264 |  |
Long-term debt (including current portion) | 4,836 | 4,835 |  |
Debt Issuance Costs, Net | 44 | 44 |  |
Fair Value |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |
Cash, cash equivalents, and restricted cash | 237 | 216 |  |
Investments, Fair Value Disclosure |  | 240 |  |
Short-term Debt, Fair Value | 1,248 | 1,070 |  |
Long-term Debt, Fair Value | 12,881 | 12,453 |  |
Fair Value | Level 1 |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |
Cash, cash equivalents, and restricted cash | 237 | 216 |  |
Investments, Fair Value Disclosure |  | 0 |  |
Short-term Debt, Fair Value | 0 | 0 |  |
Long-term Debt, Fair Value | 0 | 0 |  |
Fair Value | Level 2 |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |
Cash, cash equivalents, and restricted cash | 0 | 0 |  |
Investments, Fair Value Disclosure |  | 240 |  |
Short-term Debt, Fair Value | 1,248 | 1,070 |  |
Long-term Debt, Fair Value | 12,408 | 11,989 |  |
Fair Value | Level 3 |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |
Cash, cash equivalents, and restricted cash | 0 | 0 |  |
Investments, Fair Value Disclosure |  | 0 |  |
Short-term Debt, Fair Value | 0 | 0 |  |
Long-term Debt, Fair Value | 473 | 464 |  |
Fair Value | Union Electric Company |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |
Cash, cash equivalents, and restricted cash | 11 | 13 |  |
Investments, Fair Value Disclosure |  | 240 |  |
Short-term Debt, Fair Value | 197 | 329 |  |
Long-term Debt, Fair Value | 5,706 | 5,365 |  |
Fair Value | Union Electric Company | Level 1 |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |
Cash, cash equivalents, and restricted cash | 11 | 13 |  |
Investments, Fair Value Disclosure |  | 0 |  |
Short-term Debt, Fair Value | 0 | 0 |  |
Long-term Debt, Fair Value | 0 | 0 |  |
Fair Value | Union Electric Company | Level 2 |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |
Cash, cash equivalents, and restricted cash | 0 | 0 |  |
Investments, Fair Value Disclosure |  | 240 |  |
Short-term Debt, Fair Value | 197 | 329 |  |
Long-term Debt, Fair Value | 5,706 | 5,365 |  |
Fair Value | Union Electric Company | Level 3 |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |
Cash, cash equivalents, and restricted cash | 0 | 0 |  |
Investments, Fair Value Disclosure |  | 0 |  |
Short-term Debt, Fair Value | 0 | 0 |  |
Long-term Debt, Fair Value | 0 | 0 |  |
Fair Value | Ameren Illinois Company |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |
Cash, cash equivalents, and restricted cash | 217 | 191 |  |
Short-term Debt, Fair Value | 324 | 264 |  |
Long-term Debt, Fair Value | 4,368 | 4,320 |  |
Fair Value | Ameren Illinois Company | Level 1 |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |
Cash, cash equivalents, and restricted cash | 217 | 191 |  |
Short-term Debt, Fair Value | 0 | 0 |  |
Long-term Debt, Fair Value | 0 | 0 |  |
Fair Value | Ameren Illinois Company | Level 2 |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |
Cash, cash equivalents, and restricted cash | 0 | 0 |  |
Short-term Debt, Fair Value | 324 | 264 |  |
Long-term Debt, Fair Value | 4,368 | 4,320 |  |
Fair Value | Ameren Illinois Company | Level 3 |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |
Cash, cash equivalents, and restricted cash | 0 | 0 |  |
Short-term Debt, Fair Value | 0 | 0 |  |
Long-term Debt, Fair Value | $ 0 | $ 0 |  |

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