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Income Taxes (Tables)
 9 Months Ended
Sep. 30, 2025
Income Tax Disclosure [Abstract]  
Schedule of Effective Income Tax Rate Reconciliation
The following table presents a reconciliation of the federal statutory corporate income tax rate to the effective income tax rate for the three and nine months ended September 30, 2025 and 2024:
AmerenAmeren MissouriAmeren Illinois
202520242025202420252024
Three Months
Federal statutory corporate income tax rate21 %21 %21 %21 %21 %21 %
Increases (decreases) from:
Amortization of excess deferred taxes(a)
(7)(9)(9)(15)(4)(4)
Revaluation of excess deferred income taxes(b)
(7)— — — (14)— 
Amortization of deferred investment tax credit(1)— (1)— — — 
Renewable and other tax credits(c)
(3)(4)(7)(10)— — 
State tax5 4 3 3 7 7 
Depreciation differences(1)— (1)(1)(1)— 
Other permanent items— (1)— — 1 — 
Effective income tax rate7 %11 %6 %(2)%10 %24 %
Nine Months
Federal statutory corporate income tax rate21 %21 %21 %21 %21 %21 %
Increases (decreases) from:
Amortization of excess deferred taxes(a)
(7)(8)

(9)(14)

(4)(4)
Revaluation of excess deferred income taxes(b)
(3)— — — (4)— 
Amortization of deferred investment tax credit(1)— (1)— — — 
Renewable and other tax credits(c)
(3)(5)(7)(10)— — 
State tax5 5 3 3 7 7 
Depreciation differences(1)— (1)(1)— — 
Effective income tax rate11 %13 %6 %(1)%20 %24 %
(a)Reflects the amortization of amounts resulting from the revaluation of deferred income taxes subject to regulatory ratemaking, which are being refunded to customers. Deferred income taxes are revalued when federal or state income tax rates change, and the offset to the revaluation of deferred income taxes subject to regulatory ratemaking is recorded to a regulatory asset or liability.
(b)In 2024, the IRS issued a series of private letter rulings to another taxpayer, which provided guidance on applying IRS normalization rules to the calculation of tax benefits applicable to the ratemaking treatment related to net operating loss carryforwards. The rulings concluded that, for ratemaking purposes, net operating loss carryforwards should be reflected on a separate company basis and should not be reduced by payments received for the utilization of losses by other affiliates under a tax allocation agreement. In 2025, the FERC issued an order reflecting implementation of the rules for the other taxpayer who had a similar fact pattern as Ameren Illinois and ATXI. Accordingly, in the third quarter of 2025, Ameren and Ameren Illinois decreased income tax expense by $48 million and $23 million, respectively, to reflect the revaluation of excess deferred income tax regulatory liabilities resulting from TCJA for FERC-regulated jurisdictions pursuant to IRS guidance and recent FERC order.
(c)The benefit of the renewable credits associated with Missouri renewable energy standard compliance is refunded to customers through the RESRAM. The benefit of the renewable credits associated with the production and investment tax credit tracker will be refunded to customers based on MoPSC approval in a regulatory rate review.