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Related Party Transactions (Effects of Related-party Transactions on the Statement of Income (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2025 | Dec. 31, 2024 | Dec. 31, 2023
Related Party Transaction [Line Items] |  |  |
Operating Revenues | $ 8,799 | $ 7,623 | $ 7,500
Other operations and maintenance | 1,974 | 1,969 | 1,866
Interest Charges | 776 | 663 | 566
Ameren Missouri |  |  |
Related Party Transaction [Line Items] |  |  |
Operating Revenues | 4,795 | 3,993 | 3,859
Other operations and maintenance | 1,029 | 1,050 | 1,003
Interest Charges | 297 | 244 | 227
Ameren Missouri | Ameren Missouri Power Supply Agreements with Ameren Illinois | Related Party |  |  |
Related Party Transaction [Line Items] |  |  |
Operating Revenues | 0 | 0 | 2
Ameren Missouri | Ameren Missouri and Ameren Illinois Rent and Facility Services | Related Party |  |  |
Related Party Transaction [Line Items] |  |  |
Operating Revenues | 31 | 31 | 32
Other operations and maintenance | 1 | 1 | 1
Ameren Missouri | Ameren Missouri and Ameren Illinois miscellaneous support services and services provided to ATXI | Related Party |  |  |
Related Party Transaction [Line Items] |  |  |
Operating Revenues | 1 | 2 | 1
Ameren Missouri | Total Operating Revenues | Related Party |  |  |
Related Party Transaction [Line Items] |  |  |
Operating Revenues | 32 | 33 | 34
Ameren Missouri | Ameren Missouri Interconnection Agreement With Ameren Illinois | Related Party |  |  |
Related Party Transaction [Line Items] |  |  |
Purchased power | 1 | 1 | 0
Ameren Missouri | Ameren Illinois Transmission Services From ATXI | Related Party |  |  |
Related Party Transaction [Line Items] |  |  |
Purchased power | 10 | 9 | 2
Ameren Missouri | Total Purchased Power | Related Party |  |  |
Related Party Transaction [Line Items] |  |  |
Purchased power | 11 | 9 | 2
Ameren Missouri | Ameren Services Support Services Agreement | Related Party |  |  |
Related Party Transaction [Line Items] |  |  |
Other operations and maintenance | 177 | 169 | 148
Ameren Missouri | Total Related Party Other Operations and Maintenance | Related Party |  |  |
Related Party Transaction [Line Items] |  |  |
Other operations and maintenance | 177 | 170 | 148
Ameren Missouri | Money Pool Borrowings (Advances) | Related Party |  |  |
Related Party Transaction [Line Items] |  |  |
Interest Charges | (1) | (4) | 1
Ameren Missouri | Long-Term Debt | Related Party |  |  |
Related Party Transaction [Line Items] |  |  |
Interest Charges | (3) | 1 |
Ameren Illinois |  |  |
Related Party Transaction [Line Items] |  |  |
Operating Revenues | 3,844 | 3,472 | 3,482
Purchased power | 781 | 621 | 820
Other operations and maintenance | 945 | 906 | 818
Interest Charges | 260 | 241 | 204
Ameren Illinois | Ameren Missouri and Ameren Illinois Rent and Facility Services | Related Party |  |  |
Related Party Transaction [Line Items] |  |  |
Operating Revenues | 1 | 1 | 1
Other operations and maintenance | 3 | 1 | 3
Ameren Illinois | Ameren Missouri and Ameren Illinois miscellaneous support services and services provided to ATXI | Related Party |  |  |
Related Party Transaction [Line Items] |  |  |
Operating Revenues | 5 | 2 | 2
Ameren Illinois | Total Operating Revenues | Related Party |  |  |
Related Party Transaction [Line Items] |  |  |
Operating Revenues | 7 | 3 | 2
Ameren Illinois | Ameren Illinois Power Supply Agreements with Ameren Missouri | Related Party |  |  |
Related Party Transaction [Line Items] |  |  |
Purchased power | 0 | 0 | 2
Ameren Illinois | Ameren Illinois Interconnection Agreement With Ameren Missouri | Related Party |  |  |
Related Party Transaction [Line Items] |  |  |
Operating Revenues | 1 | 1 | 0
Ameren Illinois | Ameren Illinois Transmission Services From ATXI | Related Party |  |  |
Related Party Transaction [Line Items] |  |  |
Purchased power | 2 | 2 | 1
Ameren Illinois | Total Purchased Power | Related Party |  |  |
Related Party Transaction [Line Items] |  |  |
Purchased power | 2 | 2 | 3
Ameren Illinois | Ameren Services Support Services Agreement | Related Party |  |  |
Related Party Transaction [Line Items] |  |  |
Other operations and maintenance | 164 | 158 | 138
Ameren Illinois | Total Related Party Other Operations and Maintenance | Related Party |  |  |
Related Party Transaction [Line Items] |  |  |
Other operations and maintenance | 167 | 159 | 141
Ameren Illinois | Money Pool Borrowings (Advances) | Related Party |  |  |
Related Party Transaction [Line Items] |  |  |
Interest Charges | 1 | 1 | $ 1
Ameren Illinois | Long-Term Debt | Related Party |  |  |
Related Party Transaction [Line Items] |  |  |
Interest Charges | $ 1 | $ 1 |

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