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Income Taxes (Schedule of Effective Income Tax Rate Reconciliation) (Details) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2025 | Sep. 30, 2024 | Sep. 30, 2025 | Sep. 30, 2024
Income Taxes [Line Items] |  |  |  |
Federal statutory corporate income tax rate | 21.00% | 21.00% | 21.00% | 21.00%
Increases (decreases) from: |  |  |  |
Amortization of excess deferred taxes | (7.00%) | (9.00%) | (7.00%) | (8.00%)
Valuation allowance | (7.00%) | 0.00% | (3.00%) | 0.00%
Amortization of deferred investment tax credit | (1.00%) | 0.00% | (1.00%) | 0.00%
Renewable and other tax credits | (3.00%) | (4.00%) | (3.00%) | (5.00%)
State tax | 5.00% | 4.00% | 5.00% | 5.00%
Depreciation differences | (1.00%) | 0.00% | (1.00%) | 0.00%
Other permanent items | 0.00% | (1.00%) |  |
Effective income tax rate | 7.00% | 11.00% | 11.00% | 13.00%
Income Tax Expense (Benefit) | $ 52 | $ 57 | $ 145 | $ 140
TCJA - Private Letter Ruling |  |  |  |
Increases (decreases) from: |  |  |  |
Income Tax Expense (Benefit) |  |  | $ 48 |
Union Electric Company |  |  |  |
Income Taxes [Line Items] |  |  |  |
Federal statutory corporate income tax rate | 21.00% | 21.00% | 21.00% | 21.00%
Increases (decreases) from: |  |  |  |
Amortization of excess deferred taxes | (9.00%) | (15.00%) | (9.00%) | (14.00%)
Valuation allowance | 0.00% | 0.00% | 0.00% | 0.00%
Amortization of deferred investment tax credit | (1.00%) | 0.00% | (1.00%) | 0.00%
Renewable and other tax credits | (7.00%) | (10.00%) | (7.00%) | (10.00%)
State tax | 3.00% | 3.00% | 3.00% | 3.00%
Depreciation differences | (1.00%) | (1.00%) | (1.00%) | (1.00%)
Other permanent items | 0.00% | 0.00% |  |
Effective income tax rate | 6.00% | (2.00%) | 6.00% | (1.00%)
Income Tax Expense (Benefit) | $ 32 | $ (6) | $ 43 | $ (4)
Ameren Illinois Company |  |  |  |
Income Taxes [Line Items] |  |  |  |
Federal statutory corporate income tax rate | 21.00% | 21.00% | 21.00% | 21.00%
Increases (decreases) from: |  |  |  |
Amortization of excess deferred taxes | (4.00%) | (4.00%) | (4.00%) | (4.00%)
Valuation allowance | (14.00%) | 0.00% | (4.00%) | 0.00%
Amortization of deferred investment tax credit | 0.00% | 0.00% | 0.00% | 0.00%
Renewable and other tax credits | 0.00% | 0.00% | 0.00% | 0.00%
State tax | 7.00% | 7.00% | 7.00% | 7.00%
Depreciation differences | (1.00%) | 0.00% | 0.00% | 0.00%
Other permanent items | 1.00% | 0.00% |  |
Effective income tax rate | 10.00% | 24.00% | 20.00% | 24.00%
Income Tax Expense (Benefit) | $ 17 | $ 37 | $ 134 | $ 144
Ameren Illinois Company | TCJA - Private Letter Ruling |  |  |  |
Increases (decreases) from: |  |  |  |
Income Tax Expense (Benefit) |  |  | $ 23 |

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