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Fair Value Measurements (Tables) | 12 Months Ended
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Dec. 31, 2025
Fair Value Disclosures [Abstract] |
Schedule Of Fair Value Hierarchy Of Assets And Liabilities Measured At Fair Value On Recurring Basis | The following table sets forth, by level within the fair value hierarchy, our assets and liabilities measured at fair value on a recurring basis as of December 31, 2025 and 2024:
|  | December 31, 2025 |  |  | December 31, 2024 |
|  | Level 1 | Level 2 | Level 3 | Total |  |  | Level 1 | Level 2 | Level 3 | Total |
Assets: |  |  |  |  |  |  |  |  |  |  |
Ameren Missouri |  |  |  |  |  |  |  |  |  |  |
| Derivative assets – commodity contracts: |  |  |  |  |  |  |  |  |  |  |
| Natural gas | — |  | 1 |  | — |  | 1 |  |  |  | — |  | 4 |  | — |  | 4 |  |
| Power | — |  | — |  | 11 |  | 11 |  |  |  | — |  | — |  | 6 |  | 6 |  |
| Total derivative assets – commodity contracts | $ | — |  | $ | 1 |  | $ | 11 |  | $ | 12 |  |  |  | $ | — |  | $ | 4 |  | $ | 6 |  | $ | 10 |  |
| Nuclear decommissioning trust fund: |  |  |  |  |  |  |  |  |  |  |
| Equity securities: |  |  |  |  |  |  |  |  |  |  |
| U.S. large capitalization | $ | 1,028 |  | $ | — |  | $ | — |  | $ | 1,028 |  |  |  | $ | 911 |  | $ | — |  | $ | — |  | $ | 911 |  |
| Debt securities: |  |  |  |  |  |  |  |  |  |  |
| U.S. Treasury and agency securities | — |  | 225 |  | — |  | 225 |  |  |  | — |  | 191 |  | — |  | 191 |  |
| Corporate bonds | — |  | 177 |  | — |  | 177 |  |  |  | — |  | 145 |  | — |  | 145 |  |
| Other | — |  | 84 |  | — |  | 84 |  |  |  | — |  | 86 |  | — |  | 86 |  |
| Total nuclear decommissioning trust fund | $ | 1,028 |  | $ | 486 |  | $ | — |  | $ | 1,514 |  | (a) |  | $ | 911 |  | $ | 422 |  | $ | — |  | $ | 1,333 |  | (a)
| Total Ameren Missouri | $ | 1,028 |  | $ | 487 |  | $ | 11 |  | $ | 1,526 |  |  |  | $ | 911 |  | $ | 426 |  | $ | 6 |  | $ | 1,343 |  |
Ameren Illinois |  |  |  |  |  |  |  |  |  |  |
| Derivative assets – commodity contracts: |  |  |  |  |  |  |  |  |  |  |
| Natural gas | $ | — |  | $ | 2 |  | $ | 2 |  | $ | 4 |  |  |  | $ | — |  | $ | 3 |  | $ | 3 |  | $ | 6 |  |
| Total Ameren Illinois | $ | — |  | $ | 2 |  | $ | 2 |  | $ | 4 |  |  |  | $ | — |  | $ | 3 |  | $ | 3 |  | $ | 6 |  |
Ameren |  |  |  |  |  |  |  |  |  |  |
| Derivative assets – commodity contracts(b) | $ | — |  | $ | 3 |  | $ | 13 |  | $ | 16 |  |  |  | $ | — |  | $ | 7 |  | $ | 9 |  | $ | 16 |  |
| Nuclear decommissioning trust fund(c) | 1,028 |  | 486 |  | — |  | 1,514 |  | (a) |  | 911 |  | 422 |  | — |  | 1,333 |  | (a)
| Total Ameren | $ | 1,028 |  | $ | 489 |  | $ | 13 |  | $ | 1,530 |  |  |  | $ | 911 |  | $ | 429 |  | $ | 9 |  | $ | 1,349 |  |
Liabilities: |  |  |  |  |  |  |  |  |  |  |
Ameren Missouri |  |  |  |  |  |  |  |  |  |  |
| Derivative liabilities – commodity contracts: |  |  |  |  |  |  |  |  |  |  |
| Fuel oils | $ | 5 |  | $ | — |  | $ | — |  | $ | 5 |  |  |  | $ | 4 |  | $ | — |  | $ | — |  | $ | 4 |  |
| Natural gas | — |  | 10 |  | — |  | 10 |  |  |  | — |  | 11 |  | — |  | 11 |  |
| Total Ameren Missouri | $ | 5 |  | $ | 10 |  | $ | — |  | $ | 15 |  |  |  | $ | 4 |  | $ | 11 |  | $ | — |  | $ | 15 |  |
Ameren Illinois |  |  |  |  |  |  |  |  |  |  |
| Derivative liabilities – commodity contracts: |  |  |  |  |  |  |  |  |  |  |
| Natural gas | $ | 1 |  | $ | 28 |  | $ | 3 |  | $ | 32 |  |  |  | $ | 1 |  | $ | 28 |  | $ | 6 |  | $ | 35 |  |
| Power | — |  | 13 |  | 66 |  | 79 |  |  |  | — |  | — |  | 53 |  | 53 |  |
| Total Ameren Illinois | $ | 1 |  | $ | 41 |  | $ | 69 |  | $ | 111 |  |  |  | $ | 1 |  | $ | 28 |  | $ | 59 |  | $ | 88 |  |
Ameren |  |  |  |  |  |  |  |  |  |  |
| Derivative liabilities – commodity contracts(b) | $ | 6 |  | $ | 51 |  | $ | 69 |  | $ | 126 |  |  |  | $ | 5 |  | $ | 39 |  | $ | 59 |  | $ | 103 |  |
(a)Balance excludes $12 million and $9 million of cash and cash equivalents, receivables, payables, and accrued income, net for December 31, 2025 and 2024, respectively. (b)See the Ameren Missouri and Ameren Illinois sections of the table for the fair value of Ameren’s derivative assets and liabilities by type of commodity. (c)See the Ameren Missouri section of the table for Ameren’s nuclear decommissioning trust fund by investment type.
Schedule Of Changes In The Fair Value Of Financial Assets And Liabilities Classified As Level 3 In The Fair Value Hierarchy | The following table presents the fair value reconciliation of Level 3 power derivative contract assets and liabilities measured at fair value on a recurring basis for the years ended December 31, 2025 and 2024:
| 2025 |  |  | 2024
| Ameren Missouri | Ameren Illinois | Ameren |  |  | Ameren Missouri | Ameren Illinois | Ameren
Beginning balance at January 1 | $ | 6 |  | $ | (53) |  | $ | (47) |  |  |  | $ | 4 |  | $ | (68) |  | $ | (64) |
Realized and unrealized gains (losses) included in regulatory assets/liabilities | 29 |  | (24) |  | 5 |  |  |  | 12 |  | (1) |  | 11 |
Settlements | (24) |  | 11 |  | (13) |  |  |  | (10) |  | 16 |  | 6 |
Ending balance at December 31 | $ | 11 |  | $ | (66) |  | $ | (55) |  |  |  | $ | 6 |  | $ | (53) |  | $ | (47) |
Change in unrealized gains (losses) related to assets/liabilities held at December 31 | 11 |  | (23) |  | (12) |  |  |  | 6 |  | 3 |  | 9 |
Fair Value Inputs, Assets and Liabilities, Quantitative Information | The following table describes the valuation techniques and significant unobservable inputs utilized for the fair value of our Level 3 power derivative contract assets and liabilities as of December 31, 2025 and 2024:
|  | Fair Value |  |  |  |  | Weighted Average(b)
| Commodity | Assets | Liabilities |  | Valuation Technique(s) | Unobservable Input(a) | Range
2025 | Power(c) | $ | 11 |  | $ | (66) |  |  | Discounted cash flow | Average forward peak and off-peak pricing – forwards/swaps ($/MWh) | 33 – 72 | 43
|  |  |  |  |  | Nodal basis ($/MWh) | (9) – (2) | (5)
2024 | Power(c) | $ | 6 |  | $ | (53) |  |  | Discounted cash flow | Average forward peak and off-peak pricing – forwards/swaps ($/MWh) | 32 – 69 | 45
|  |  |  |  |  | Nodal basis ($/MWh) | (8) – (2) | (5)
(a)Generally, significant increases (decreases) in these inputs in isolation would result in a significantly higher (lower) fair value measurement. (b)Unobservable inputs were weighted by relative fair value. (c)Valuations use visible forward prices adjusted for nodal-to-hub basis differentials.
Schedule of Carrying Values and Estimated Fair Values of Debt Instruments [Table Text Block] | The following table sets forth the carrying amount and, by level within the fair value hierarchy, the fair value of long-term debt (including current portion), disclosed, but not recorded, at fair value as of December 31, 2025 and 2024:
| Carrying Amount(a) |  |  | Fair Value |  |
Long-Term Debt (Including Current Portion): |  |  |  | Level 2 |  | Level 3 |  | Total
| December 31, 2025
Ameren(b) | $ | 19,187 |  |  |  |  | $ | 17,433 |  |  | $ | 559 |  | (c) | $ | 17,992 |
Ameren Missouri(d) | 8,230 |  |  |  |  | 7,608 |  |  | — |  |  | 7,608 |
Ameren Illinois(d) | 6,259 |  |  |  |  | 5,753 |  |  | — |  |  | 5,753 |
| December 31, 2024
Ameren(b) | $ | 17,579 |  |  |  |  | $ | 15,395 |  |  | $ | 538 |  | (c) | $ | 15,933 |
Ameren Missouri(d) | 7,745 |  |  |  |  | 6,926 |  |  | — |  |  | 6,926 |
Ameren Illinois(d) | 5,852 |  |  |  |  | 5,243 |  |  | — |  |  | 5,243 |
(a)Included unamortized debt issuance costs, which were excluded from the fair value measurement, of $136 million, $62 million, and $56 million for Ameren, Ameren Missouri, and Ameren Illinois, respectively, as of December 31, 2025. Included unamortized debt issuance costs, which were excluded from the fair value measurement, of $129 million, $62 million, and $51 million for Ameren, Ameren Missouri, and Ameren Illinois, respectively, as of December 31, 2024. (b)Amount excludes Ameren (parent)’s repurchase of Ameren Missouri’s senior secured notes and first mortgage bonds and Ameren Illinois’ first mortgage bonds that were accounted for as a debt extinguishment. See Note 5 – Long-term Debt and Equity Financings for additional information. (c)The Level 3 fair value amount consists of ATXI’s senior unsecured notes. (d)Amount includes Ameren Missouri’s senior secured notes and first mortgage bonds and Ameren Illinois’ first mortgage bonds that were repurchased by Ameren (parent) in 2025 and 2024.

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- Definition Tabular disclosure of input and valuation technique used to measure fair value and change in valuation approach and technique used to measure similar asset in prior period by class of asset or liability on non-recurring basis. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (bbb) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482106/820-10-50-2 + Details
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- Definition Tabular disclosure of assets and liabilities, including [financial] instruments measured at fair value that are classified in stockholders' equity, if any, that are measured at fair value on a recurring basis. The disclosures contemplated herein include the fair value measurements at the reporting date by the level within the fair value hierarchy in which the fair value measurements in their entirety fall, segregating fair value measurements using quoted prices in active markets for identical assets (Level 1), significant other observable inputs (Level 2), and significant unobservable inputs (Level 3). + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482106/820-10-50-2 Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482106/820-10-50-2 + Details
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