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Related Party Transactions (Tables) | 12 Months Ended
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Dec. 31, 2025
Related Party Transactions [Abstract] |
Schedule of Affiliate Receivables and Payables | The following table presents the affiliate balances related to income taxes for Ameren Missouri and Ameren Illinois as of December 31, 2025 and 2024:
| 2025 |  |  | 2024
| Ameren Missouri | Ameren Illinois |  |  | Ameren Missouri | Ameren Illinois
Income taxes payable to parent(a) | $ | — |  | $ | 4 |  |  |  | $ | — |  | $ | 32 |
Income taxes receivable from parent(b) | 3 |  | 7 |  |  |  | 28 |  | — |
(a)Included in “Accounts payable – affiliates” on the balance sheet. (b)Included in “Accounts receivable – affiliates” on the balance sheet
Schedule of Capital Contributions | The following table presents cash capital contributions received from Ameren (parent) by Ameren Missouri and Ameren Illinois for the years ended December 31, 2025, 2024, and 2023:
| 2025 |  | 2024 |  | 2023 |
Ameren Missouri(a) | $ | 28 |  |  | $ | 476 |  |  | $ | — |  |
Ameren Illinois(a) | 2 |  |  | 36 |  |  | 91 |  |
(a)Includes capital contributions made as a result of the tax allocation agreement.
Schedule of Related Party Transactions | The following table presents the impact on Ameren Missouri and Ameren Illinois of related-party transactions for the years ended December 31, 2025, 2024, and 2023. It is based primarily on the agreements discussed above and the money pool arrangements discussed in Note 4 – Short-term Debt and Liquidity.
Agreement | Income Statement Line Item |  |  |  | Ameren Missouri |  | Ameren Illinois
Ameren Missouri power supply agreements | Operating Revenues |  | 2025 | $ | — |  | $ | (a)
with Ameren Illinois |  |  | 2024 |  | — |  |  | (a)
|  |  | 2023 |  | 2 |  |  | (a)
Ameren Missouri and Ameren Illinois | Operating Revenues |  | 2025 |  | 31 |  |  | 1 |
rent and facility services |  |  | 2024 |  | 31 |  |  | 1 |
|  |  | 2023 |  | 32 |  |  | (b)
Ameren Illinois interconnection agreement | Operating Revenues |  | 2025 |  | (a) |  | 1 |
with Ameren Missouri |  |  | 2024 |  | (a) |  | (b)
|  |  | 2023 |  | (a) |  | — |
Ameren Missouri and Ameren Illinois | Operating Revenues |  | 2025 |  | 1 |  |  | 5 |
miscellaneous support services |  |  | 2024 |  | 2 |  |  | 2 |
|  |  | 2023 |  | (b) |  | 2 |
Total Operating Revenues |  |  | 2025 | $ | 32 |  | $ | 7 |
|  |  | 2024 |  | 33 |  |  | 3 |
|  |  | 2023 |  | 34 |  |  | 2 |
Ameren Illinois power supply | Purchased Power |  | 2025 | $ | (a) | $ | — |
agreements with Ameren Missouri |  |  | 2024 |  | (a) |  | — |
|  |  | 2023 |  | (a) |  | 2 |
Ameren Missouri interconnection agreement | Purchased Power |  | 2025 |  | 1 |  |  | (a)
with Ameren Illinois |  |  | 2024 |  | (b) |  | (a)
|  |  | 2023 |  | — |  |  | (a)
Ameren Missouri and Ameren Illinois | Purchased Power |  | 2025 |  | 10 |  |  | 2 |
transmission services from ATXI |  |  | 2024 |  | 9 |  |  | 2 |
|  |  | 2023 |  | 2 |  |  | 1 |
Total Purchased Power |  |  | 2025 | $ | 11 |  | $ | 2 |
|  |  | 2024 |  | 9 |  |  | 2 |
|  |  | 2023 |  | 2 |  |  | 3 |
Ameren Missouri and Ameren Illinois | Other Operations and |  | 2025 | $ | (b) | $ | 3 |
rent and facility services | Maintenance |  | 2024 |  | 1 |  |  | 1 |
|  |  | 2023 |  | (b) |  | 3 |
Ameren Services support services | Other Operations and |  | 2025 |  | 177 |  |  | 164 |
agreement | Maintenance |  | 2024 |  | 169 |  |  | 158 |
|  |  | 2023 |  | 148 |  |  | 138 |
Total Other Operations and |  |  | 2025 | $ | 177 |  | $ | 167 |
Maintenance Expenses |  |  | 2024 |  | 170 |  |  | 159 |
|  |  | 2023 |  | 148 |  |  | 141 |
Money pool borrowings (advances) | (Interest Charges) |  | 2025 | $ | (1) |  | $ | 1 |
| Other Income, Net |  | 2024 |  | (4) |  |  | (b)
|  |  | 2023 |  | (b) |  | (b)
Long-term debt, net - related parties | (Interest Charges) |  | 2025 | $ | (3) |  | $ | (b)
|  |  | 2024 |  | (b) |  | (b)
|  |  | 2023 |  | (a) |  | (a)
(a)Not applicable. (b)Amount less than $1 million.

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- Definition Tabular disclosure of related party transactions. Examples of related party transactions include, but are not limited to, transactions between (a) a parent company and its subsidiary; (b) subsidiaries of a common parent; (c) and entity and its principal owners and (d) affiliates. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 850 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (d) -Publisher FASB -URI https://asc.fasb.org/1943274/2147483326/850-10-50-1 Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 850 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (c) -Publisher FASB -URI https://asc.fasb.org/1943274/2147483326/850-10-50-1 + Details
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