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Rate And Regulatory Matters (Tables)
 12 Months Ended
Dec. 31, 2025
Public Utilities, General Disclosures [Abstract]  
Schedule of Regulatory Frameworks and Significant Recovery Mechanisms
The following table presents the regulatory frameworks and significant regulatory recovery mechanisms for each of Ameren’s rate-regulated businesses, which are discussed in more detail below:
Ameren MissouriAmeren Illinois’ electric distribution businessAmeren Illinois’ natural gas delivery businessAmeren Illinois’ and ATXI’s electric transmission businesses
Regulatory framework
•Historical test year ratemaking
•Natural gas revenues for residential customers adjusted for sales volume deviations resulting from weather through the WNAR
•MYRP
•Initial rates based on future test years
•Revenues decoupled from sales volumes and wholesale and miscellaneous revenues through the RBA
•Future test year ratemaking
•Revenues for residential and small nonresidential customers decoupled from sales volumes through the VBA
•Formula ratemaking
•Initial rates based on future test year
•Revenues decoupled from sales volumes
Regulatory mechanisms
•PISA

Riders:
•RESRAM
•FAC
•Rush Island securitization
•MEEIA
•PGA
•WNAR

Trackers:
•Pension and postretirement benefit costs
•Certain excess deferred income taxes
•Property taxes
•Production and investment tax credits or proceeds from the sale of certain tax credits allowed under the IRA
•Renewable solutions program revenues and costs(a)
•Electric distribution service and energy-efficiency revenue requirement reconciliation adjustments(b)

Riders:
•RBA
•Power procurement
•Transmission services
•Renewable energy credit compliance
•Zero emission credits
•Customer generation rebate program costs
•Certain environmental costs
•Bad debt write-offs
•Costs of certain asbestos-related claims
Riders:
•PGA
•VBA
•Energy-efficiency program costs
•Certain environmental costs
•Bad debt write-offs
•Invested capital taxes
•Revenue requirement reconciliation adjustment
(a)Ameren Missouri’s renewable solutions program allows certain commercial, industrial, and governmental customers who enroll in the program to receive up to 100% of their energy from renewable resources.
(b)Reconciliation adjustments under an MYRP are subject to a reconciliation cap which limits annual adjustment to 105%. See below for additional information regarding the reconciliation cap.
Schedule of Energy Center Projects The following table provides information with respect to each facility:
Agreement typeFacility sizeStatus of MoPSC CCN
In-service date(a)
Vandalia Solar Project(b)
Self-build
50-MW
Approved March 2024December 2025
Bowling Green Solar Project(b)
Self-build
50-MW
Approved March 2024First quarter 2026
Split Rail Solar Project
Build-transfer(c)
300-MW
Approved March 2024Second quarter 2026
Castle Bluff Natural Gas Project(d)
Self-build
800-MW
Approved October 2024Fourth quarter 2027
Big Hollow Battery Energy Storage Project(e)
Self-build
400-MW
Approved February 2026Second quarter 2028
Big Hollow Natural Gas Project(e)
Self-build
800-MW
Approved February 2026Third quarter 2028
Reform Solar ProjectSelf-build
250-MW
Filed August 2025(f)
Fourth quarter 2028
(a)In-service dates are dependent on the timing of regulatory approvals and construction completion, among other things.
(b)These projects collectively represent approximately $0.2 billion of capital expenditures.
(c)Ameren Missouri received FERC approval for the acquisition in November 2024. In February 2026, Ameren Missouri acquired the Split Rail Solar Project, which includes solar panels, project design, land rights, and engineering, procurement, and construction agreements, for approximately $0.6 billion, and took over construction management of the project.
(d)This project represents approximately $0.9 billion of capital expenditures.
(e)These projects represent approximately $2 billion of capital expenditures.
(f)Ameren Missouri expects a decision by the MoPSC in the first half of 2026.
Schedule Of Regulatory Assets And Liabilities
The following table presents our regulatory assets and regulatory liabilities at December 31, 2025 and 2024:
20252024
Ameren
Missouri		Ameren
Illinois		AmerenAmeren
Missouri		Ameren
Illinois		Ameren
Regulatory assets:
Under-recovered FAC(a)
$137 $— $137 $41 $— $41 
MTM derivative losses(b)
15 111 126 15 88 103 
IEIMA revenue requirement reconciliation adjustment(c)(d)
— — — — 139 139 
MYRP revenue requirement reconciliation adjustment(d)(e)
— 74 74 — 24 24 
Under-recovered RBA(f)
— 29 29 — 22 22 
FERC revenue requirement reconciliation adjustment(g)
— 45 73 — 55 90 
Under-recovered VBA(h)
— 21 21 — 49 49 
Income taxes(i)
282 91 376 237 81 322 
Bad debt rider(j)
— 13 13 — 25 25 
Callaway refueling and maintenance outage costs(k)
32 — 32 13 — 13 
Unamortized loss on reacquired debt(l)
40 4 44 42 5 47 
Environmental cost riders(m)
— 46 46 — 43 43 
Storm costs(d)(n)
— 15 15 — 18 18 
Customer generation rebate program(d)(o)
— 141 141 — 89 89 
PISA(d)(p)
558 — 558 464 — 464 
Rush Island Energy Center securitization(q)
443 — 443 465 — 465 
RESRAM(r)
44 — 44 51 — 51 
Certain Meramec Energy Center costs(s)
14 — 14 26 — 26 
Energy-efficiency rider(d)(t)
— 624 624 — 576 576 
Property tax tracker(u)
18 — 18 22 — 22 
Other48 34 83 56 78 134 
Total regulatory assets$1,631 $1,248 $2,911 $1,432 $1,292 $2,763 
Less: current regulatory assets(181)(189)(387)(66)(281)(366)
Noncurrent regulatory assets$1,450 $1,059 $2,524 $1,366 $1,011 $2,397 
Regulatory liabilities:
Over-recovered Illinois electric power costs(v)
— 85 85 — 34 34 
Over-recovered PGA(v)
3 39 42 2 33 35 
MTM derivative gains(b)
12 4 16 10 6 16 
Income taxes(i)
963 546 1,587 1,040 679 1,804 
Cost of removal(w)
1,203 1,230 2,511 1,118 1,115 2,294 
AROs(x)
841 — 841 691 — 691 
Pension and postretirement benefit costs(y)
230 237 467 202 156 358 
Pension and postretirement benefit costs tracker(z)
9 — 9 70 — 70 
Renewable energy credits and zero emission credits(aa)
— 699 699 — 586 586 
Certain Rush Island Energy Center costs(ab)
31 — 31 66 — 66 
Rush Island Energy Center base rate revenue deferral(ac)
31 — 31 13 — 13 
Other24 67 94 1 43 50 
Total regulatory liabilities$3,347 $2,907 $6,413 $3,213 $2,652 $6,017 
Less: current regulatory liabilities(23)(132)(158)(37)(79)(120)
Noncurrent regulatory liabilities$3,324 $2,775 $6,255 $3,176 $2,573 $5,897 
(a)Under-recovered fuel and purchased power costs to be recovered through the FAC. Specific accumulation periods aggregate the under-recovered costs over four months, any related adjustments that occur over the following four months, and the recovery from customers that occurs over the next eight months.
(b)Deferral of commodity-related derivative MTM losses or gains. See Note 7 – Derivative Financial Instruments for additional information.
(c)The difference between Ameren Illinois’ electric distribution service annual revenue requirement calculated under the IEIMA performance-based formula ratemaking framework and the revenue requirement included in customer rates for that year. The under-recovery was recovered from customers with a return at the applicable WACC in 2025.
(d)These assets earn a return at the applicable WACC.
(e)The difference between Ameren Illinois' actual annual electric distribution revenue requirement, as adjusted for certain cost variations, and the ICC-approved revenue requirement, subject to a reconciliation cap. The under-recovery will be recovered from customers with a return at the applicable WACC within two years.
(f)Under-recovered electric distribution service revenue caused by sales volume and/or wholesale and miscellaneous revenue deviations from the related revenue requirement approved by the ICC for a given year. The under-recovery will be recovered from customers within two years.
(g)Ameren Illinois’ and ATXI’s annual revenue requirement reconciliation calculated pursuant to the FERC’s electric transmission formula ratemaking framework. Any under-recovery or over-recovery will be recovered from, or refunded to, customers within two years.
(h)Under-recovered natural gas revenue caused by sales volume deviations from weather normalized sales approved by the ICC in regulatory rate reviews. Each year’s amount will be recovered from customers from April through December of the following year.
(i)The regulatory assets represent amounts that will be recovered from customers for deferred income taxes related to the equity component of allowance for funds used during construction, the securitization of the Rush Island Energy Center, and the effects of tax rate increases. The regulatory liabilities represent amounts that will be refunded to customers for excess deferred income taxes related to depreciation differences caused by a decrease in the statutory rates, other tax liabilities, and amounts related to the unamortized portion of investment tax credits. Amounts associated with the equity component of allowance for funds used during construction, the securitization of the Rush Island Energy Center, and amounts related to the unamortized portion of investment tax credits will be amortized over the expected life of the related assets. For net regulatory liabilities related to deferred income taxes recorded at rates other than the current statutory rate, the weighted-average remaining amortization periods at Ameren, Ameren Missouri, and Ameren Illinois are 38, 30, and 44 years. In addition, the regulatory liabilities for Ameren Missouri include a regulatory recovery mechanism for the difference between production and investment tax credits or proceeds from the sale of such tax credits allowed under the IRA and the level of such tax credits included in customer rates. The period of refund varies based on MoPSC approval in a regulatory rate review. Amounts included in the accumulation period approved in the April 2025 MoPSC electric rate order, discussed above, are being amortized over five years beginning June 2025. An amortization period for subsequent accumulations will be established in a future rate review.
(j)A rider for the difference between the level of bad debt write-offs, net of any subsequent recoveries, incurred by Ameren Illinois and the level of such costs included in electric distribution and natural gas delivery service rates. Under-recovered or over-recovered costs for each year are collected from, or refunded to, customers over a twelve-month period beginning in June of the following year.
(k)Maintenance expenses related to scheduled refueling and maintenance outages at Ameren Missouri’s Callaway Energy Center. Amounts are amortized over the period between refueling and maintenance outages, which has historically been approximately 18 months.
(l)Losses related to reacquired debt. These amounts are being amortized over the lives of the related new debt issuances or the original lives of the old debt issuances if no new debt was issued.
(m)The recoverable portion of accrued environmental site liabilities that will be collected from electric and natural gas customers through ICC-approved cost recovery riders. The period of recovery will depend on the timing of remediation expenditures. See Note 14 – Commitments and Contingencies for additional information.
(n)Storm costs from 2021 through 2025 deferred in accordance with the IEIMA and MYRP. These costs are being amortized over five-year periods beginning in the year the storm occurred.
(o)Costs associated with Ameren Illinois’ customer generation rebate program. Costs are amortized over a 15-year period, beginning in the year rebates are paid.
(p)Under the PISA, Ameren Missouri is permitted to defer and recover 85% of the depreciation expense and earn a return at the applicable WACC on 85% of investments in certain property, plant, and equipment placed in service and not included in base rates. Accumulated PISA deferrals, which also earn a return at the applicable WACC, are added to rate base prospectively and amortized over a period of 20 years following a regulatory rate review.
(q)In June 2024, the MoPSC issued a financing order authorizing the issuance of securitized utility tariff bonds by AMF to finance costs related to the accelerated retirement of the Rush Island Energy Center, which includes the remaining unrecovered net plant balance associated with the facility, among other costs. Ameren Missouri is collecting the amounts necessary to repay the securitized utility tariff bonds over approximately 15 years beginning in December 2024.
(r)Under-recovered costs associated with Ameren Missouri’s compliance with the state of Missouri’s renewable energy standard. Under-recovered or over-recovered costs are aggregated over a twelve-month period beginning each August and are amortized over a twelve-month period beginning in February of the following year.
(s)Certain costs associated with the Meramec Energy Center, which were authorized for recovery by a December 2021 MoPSC electric rate order. These costs are being collected over five years beginning in February 2022.
(t)The electric energy-efficiency investments are being amortized over their weighted-average useful lives beginning in the period in which they were made, with current remaining amortization periods ranging from one to 13 years.
(u)A regulatory recovery mechanism for the difference between actual property taxes incurred by Ameren Missouri and the related taxes included in customer rates. The period of recovery, or refund, varies based on MoPSC approval in a regulatory rate review. Electric amounts accumulated through 2024 are being amortized over three years beginning June 2025. Gas amounts accumulated through 2024 are being amortized over five years beginning September 2025. For electric and natural gas related costs incurred subsequent to 2024, the amortization period will be determined in a future regulatory rate review.
(v)Over-recovered costs from utility customers. Amounts will be refunded to customers within one year of the deferral.
(w)Estimated funds collected from customers to pay for the future removal cost of property, plant, and equipment when retired from service.
(x)The ARO regulatory liability includes the nuclear decommissioning trust fund balance ($1,526 million and $1,342 million at December 31, 2025 and 2024, respectively), net of recoverable removal costs for AROs ($685 million and $651 million at December 31, 2025 and 2024, respectively). See Note 1 – Summary of Significant Accounting Policies – Asset Retirement Obligations and Removal Costs.
(y)Over-recovered costs are being amortized in proportion to the recognition of prior service costs (credits) and actuarial losses (gains) attributable to Ameren’s pension plan and postretirement benefit plans. See Note 10 – Retirement Benefits for additional information.
(z)A regulatory recovery mechanism for the difference between the level of pension and postretirement benefit costs incurred by Ameren Missouri and the level of such costs included in customer rates. The period of refund varies based on MoPSC approval in a regulatory rate review. For electric and natural gas related costs incurred through 2024, the weighted-average remaining amortization period is four years. For electric and natural gas related costs incurred subsequent to 2024, the amortization period will be determined in a future regulatory rate review.
(aa)Funds collected for the purchase of renewable energy credits and zero emission credits through IPA procurements. The balance will be amortized as the credits are purchased. Pursuant to the CEJA, if funds collected from customers are not used to procure renewable energy credits, they would be refunded to customers pursuant to an annual reconciliation proceeding, the latest of which was approved by the ICC in May 2025 and did not result in refunds to customers.
(ab)Funds collected from the issuance of securitized utility tariff bonds by AMF primarily to pay for the decommissioning of the Rush Island Energy Center. The amortization period for the difference between the estimated costs and the actual costs incurred will be determined in a future regulatory rate review.
(ac)Base rate revenues related to the Rush Island Energy Center collected after the energy center was retired in October 2024, which is being refunded to customers over three years beginning June 2025.