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Derivative Financial Instruments (Tables) | 12 Months Ended
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Dec. 31, 2025
Derivative Instruments and Hedging Activities Disclosure [Abstract] |
Open Gross Derivative Volumes By Commodity Type | The following table presents open gross commodity contract volumes by commodity type for derivative assets and liabilities as of December 31, 2025 and 2024. As of December 31, 2025, these contracts extended through October 2029, October 2031, and May 2032 for fuel oils, natural gas, and power, respectively.
| Quantity (in millions, except as indicated)
| 2025 | 2024
Commodity | Ameren Missouri | Ameren Illinois | Ameren | Ameren Missouri | Ameren Illinois | Ameren
Fuel oils (in gallons) | 25 |  | — |  | 25 |  | 23 |  | — |  | 23 |
Natural gas (in mmbtu) | 46 |  | 217 |  | 263 |  | 45 |  | 213 |  | 258 |
Power (in MWhs) | — |  | 7 |  | 7 |  | — |  | 4 |  | 4 |
Derivative Instruments Carrying Value | The following table presents the carrying value and balance sheet location of all derivative commodity contracts, none of which were designated as hedging instruments because all qualify for regulatory deferral, as of December 31, 2025 and 2024:
| 2025 |  |  | 2024
Commodity | Balance Sheet Location |  | Ameren Missouri |  |  | Ameren Illinois |  |  | Ameren |  |  |  | Ameren Missouri |  |  | Ameren Illinois |  |  | Ameren
Natural gas | Other current assets |  | $ | 1 |  |  |  | $ | 1 |  |  |  | $ | 2 |  |  |  |  | $ | 2 |  |  |  | $ | 2 |  |  |  | $ | 4 |
| Other assets |  | — |  |  |  | 3 |  |  |  | 3 |  |  |  |  | 2 |  |  |  | 4 |  |  |  | 6 |
Power | Other current assets |  | 11 |  |  |  | — |  |  |  | 11 |  |  |  |  | 6 |  |  |  | — |  |  |  | 6 |
| Total assets |  | $ | 12 |  |  |  | $ | 4 |  |  |  | $ | 16 |  |  |  |  | $ | 10 |  |  |  | $ | 6 |  |  |  | $ | 16 |
Fuel oils | Other current liabilities |  | $ | 3 |  |  |  | $ | — |  |  |  | $ | 3 |  |  |  |  | $ | 2 |  |  |  | $ | — |  |  |  | $ | 2 |
| Other deferred credits and liabilities |  | 2 |  |  |  | — |  |  |  | 2 |  |  |  |  | 2 |  |  |  | — |  |  |  | 2 |
Natural gas | Other current liabilities |  | 5 |  |  |  | 21 |  |  |  | 26 |  |  |  |  | 5 |  |  |  | 22 |  |  |  | 27 |
| Other deferred credits and liabilities |  | 5 |  |  |  | 11 |  |  |  | 16 |  |  |  |  | 6 |  |  |  | 13 |  |  |  | 19 |
Power | Other current liabilities |  | — |  |  |  | 24 |  |  |  | 24 |  |  |  |  | — |  |  |  | 10 |  |  |  | 10 |
| Other deferred credits and liabilities |  | — |  |  |  | 55 |  |  |  | 55 |  |  |  |  | — |  |  |  | 43 |  |  |  | 43 |
| Total liabilities |  | $ | 15 |  |  |  | $ | 111 |  |  |  | $ | 126 |  |  |  |  | $ | 15 |  |  |  | $ | 88 |  |  |  | $ | 103 |

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