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Fair Value Measurements | 12 Months Ended
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Dec. 31, 2025
Fair Value Disclosures [Abstract] |
FAIR VALUE MEASUREMENTS | FAIR VALUE MEASUREMENTS Fair value is defined as the price that would be received for an asset or paid to transfer a liability (an exit price) in the principal or most advantageous market for the asset or liability in an orderly transaction between market participants on the measurement date. We use various methods to determine fair value, including market, income, and cost approaches. With these approaches, we adopt certain assumptions that market participants would use in pricing the asset or liability, including assumptions about market risk or the risks inherent in the inputs to the valuation. Inputs to valuation can be readily observable, market-corroborated, or unobservable. We use valuation techniques that maximize the use of observable inputs and minimize the use of unobservable inputs. Authoritative accounting guidance established a fair value hierarchy that prioritizes the inputs used to measure fair value. All financial assets and liabilities carried at fair value are classified and disclosed in one of the following three hierarchy levels: Level 1 (quoted prices in active markets for identical assets or liabilities): Inputs based on quoted prices in active markets for identical assets or liabilities. Level 1 assets and liabilities are primarily exchange-traded derivatives, cash and cash equivalents, and listed equity securities. The market approach is used to measure the fair value of equity securities held in Ameren Missouri’s nuclear decommissioning trust fund. Equity securities in this fund are representative of the S&P 500 index, excluding securities of Ameren Corporation, owners and/or operators of nuclear power plants, and the trustee and investment managers. The S&P 500 index comprises stocks of large-capitalization companies. Level 2 (significant other observable inputs): Market-based inputs corroborated by third-party brokers or exchanges based on transacted market data. Level 2 assets and liabilities include certain assets held in Ameren Missouri’s nuclear decommissioning trust fund, including United States Treasury and agency securities, corporate bonds and other fixed-income securities, and certain over-the-counter derivative instruments, including natural gas and financial power transactions. Fixed income securities are valued by using prices from independent industry-recognized data vendors who provide values that are either exchange-based or matrix-based. The fair value measurements of fixed-income securities classified as Level 2 are based on inputs other than quoted prices that are observable for the asset or liability. Examples are matrix pricing, market corroborated pricing, and inputs such as yield curves and indices. Derivative instruments classified as Level 2 are valued by corroborated observable inputs, such as pricing services or prices from similar instruments that trade in liquid markets. Our development and corroboration process entails obtaining multiple quotes or prices from outside sources. To derive our forward view to price our derivative instruments at fair value, we average the bid/ask spreads to the midpoints. Additionally, a review of all sources is performed to identify any anomalies or potential errors. Further, we consider the volume of transactions on certain trading platforms in our reasonableness assessment of the averaged midpoints. The value of natural gas derivative contracts is based upon exchange closing prices without significant unobservable adjustments. The value of power derivative contracts is based upon exchange closing prices or the use of multiple forward prices provided by third parties. Level 3 (significant other unobservable inputs): Unobservable inputs that are not corroborated by market data. Level 3 assets and liabilities are valued by internally developed models and assumptions or methodologies that use significant unobservable inputs. Level 3 assets and liabilities include derivative instruments that trade in less liquid markets, where pricing is largely unobservable. We value Level 3 instruments by using pricing models with inputs that are often unobservable in the market, such as certain internal assumptions, quotes or prices from outside sources not supported by a liquid market, or trend rates. We perform an analysis each quarter to determine the appropriate hierarchy level of the assets and liabilities subject to fair value measurements. Financial assets and liabilities are classified in their entirety according to the lowest level of input that is significant to the fair value measurement. All assets and liabilities whose fair value measurement is based on significant unobservable inputs are classified as Level 3. We consider nonperformance risk in our valuation of derivative instruments by analyzing our own credit standing and the credit standing of our counterparties, and by considering any credit enhancements (e.g., collateral). Included in our valuation, and based on current market conditions, is a valuation adjustment for counterparty default derived from market data such as the price of credit default swaps, bond yields, and credit ratings. No material gains or losses related to valuation adjustments for counterparty default risk were recorded at Ameren, Ameren Missouri, or Ameren Illinois in 2025, 2024, or 2023. At December 31, 2025 and 2024, the counterparty default risk valuation adjustment related to derivative contracts was immaterial for Ameren, Ameren Missouri, and Ameren Illinois. The following table sets forth, by level within the fair value hierarchy, our assets and liabilities measured at fair value on a recurring basis as of December 31, 2025 and 2024:
|  | December 31, 2025 |  |  | December 31, 2024 |
|  | Level 1 | Level 2 | Level 3 | Total |  |  | Level 1 | Level 2 | Level 3 | Total |
Assets: |  |  |  |  |  |  |  |  |  |  |
Ameren Missouri |  |  |  |  |  |  |  |  |  |  |
| Derivative assets – commodity contracts: |  |  |  |  |  |  |  |  |  |  |
| Natural gas | — |  | 1 |  | — |  | 1 |  |  |  | — |  | 4 |  | — |  | 4 |  |
| Power | — |  | — |  | 11 |  | 11 |  |  |  | — |  | — |  | 6 |  | 6 |  |
| Total derivative assets – commodity contracts | $ | — |  | $ | 1 |  | $ | 11 |  | $ | 12 |  |  |  | $ | — |  | $ | 4 |  | $ | 6 |  | $ | 10 |  |
| Nuclear decommissioning trust fund: |  |  |  |  |  |  |  |  |  |  |
| Equity securities: |  |  |  |  |  |  |  |  |  |  |
| U.S. large capitalization | $ | 1,028 |  | $ | — |  | $ | — |  | $ | 1,028 |  |  |  | $ | 911 |  | $ | — |  | $ | — |  | $ | 911 |  |
| Debt securities: |  |  |  |  |  |  |  |  |  |  |
| U.S. Treasury and agency securities | — |  | 225 |  | — |  | 225 |  |  |  | — |  | 191 |  | — |  | 191 |  |
| Corporate bonds | — |  | 177 |  | — |  | 177 |  |  |  | — |  | 145 |  | — |  | 145 |  |
| Other | — |  | 84 |  | — |  | 84 |  |  |  | — |  | 86 |  | — |  | 86 |  |
| Total nuclear decommissioning trust fund | $ | 1,028 |  | $ | 486 |  | $ | — |  | $ | 1,514 |  | (a) |  | $ | 911 |  | $ | 422 |  | $ | — |  | $ | 1,333 |  | (a)
| Total Ameren Missouri | $ | 1,028 |  | $ | 487 |  | $ | 11 |  | $ | 1,526 |  |  |  | $ | 911 |  | $ | 426 |  | $ | 6 |  | $ | 1,343 |  |
Ameren Illinois |  |  |  |  |  |  |  |  |  |  |
| Derivative assets – commodity contracts: |  |  |  |  |  |  |  |  |  |  |
| Natural gas | $ | — |  | $ | 2 |  | $ | 2 |  | $ | 4 |  |  |  | $ | — |  | $ | 3 |  | $ | 3 |  | $ | 6 |  |
| Total Ameren Illinois | $ | — |  | $ | 2 |  | $ | 2 |  | $ | 4 |  |  |  | $ | — |  | $ | 3 |  | $ | 3 |  | $ | 6 |  |
Ameren |  |  |  |  |  |  |  |  |  |  |
| Derivative assets – commodity contracts(b) | $ | — |  | $ | 3 |  | $ | 13 |  | $ | 16 |  |  |  | $ | — |  | $ | 7 |  | $ | 9 |  | $ | 16 |  |
| Nuclear decommissioning trust fund(c) | 1,028 |  | 486 |  | — |  | 1,514 |  | (a) |  | 911 |  | 422 |  | — |  | 1,333 |  | (a)
| Total Ameren | $ | 1,028 |  | $ | 489 |  | $ | 13 |  | $ | 1,530 |  |  |  | $ | 911 |  | $ | 429 |  | $ | 9 |  | $ | 1,349 |  |
Liabilities: |  |  |  |  |  |  |  |  |  |  |
Ameren Missouri |  |  |  |  |  |  |  |  |  |  |
| Derivative liabilities – commodity contracts: |  |  |  |  |  |  |  |  |  |  |
| Fuel oils | $ | 5 |  | $ | — |  | $ | — |  | $ | 5 |  |  |  | $ | 4 |  | $ | — |  | $ | — |  | $ | 4 |  |
| Natural gas | — |  | 10 |  | — |  | 10 |  |  |  | — |  | 11 |  | — |  | 11 |  |
| Total Ameren Missouri | $ | 5 |  | $ | 10 |  | $ | — |  | $ | 15 |  |  |  | $ | 4 |  | $ | 11 |  | $ | — |  | $ | 15 |  |
Ameren Illinois |  |  |  |  |  |  |  |  |  |  |
| Derivative liabilities – commodity contracts: |  |  |  |  |  |  |  |  |  |  |
| Natural gas | $ | 1 |  | $ | 28 |  | $ | 3 |  | $ | 32 |  |  |  | $ | 1 |  | $ | 28 |  | $ | 6 |  | $ | 35 |  |
| Power | — |  | 13 |  | 66 |  | 79 |  |  |  | — |  | — |  | 53 |  | 53 |  |
| Total Ameren Illinois | $ | 1 |  | $ | 41 |  | $ | 69 |  | $ | 111 |  |  |  | $ | 1 |  | $ | 28 |  | $ | 59 |  | $ | 88 |  |
Ameren |  |  |  |  |  |  |  |  |  |  |
| Derivative liabilities – commodity contracts(b) | $ | 6 |  | $ | 51 |  | $ | 69 |  | $ | 126 |  |  |  | $ | 5 |  | $ | 39 |  | $ | 59 |  | $ | 103 |  |
(a)Balance excludes $12 million and $9 million of cash and cash equivalents, receivables, payables, and accrued income, net for December 31, 2025 and 2024, respectively. (b)See the Ameren Missouri and Ameren Illinois sections of the table for the fair value of Ameren’s derivative assets and liabilities by type of commodity. (c)See the Ameren Missouri section of the table for Ameren’s nuclear decommissioning trust fund by investment type. See Note 10 – Retirement Benefits for tables that set forth, by level within the fair value hierarchy, Ameren’s pension and postretirement plan assets as of December 31, 2025 and 2024. Level 3 natural gas derivative contract assets and liabilities measured at fair value on a recurring basis were immaterial for all periods presented. The following table presents the fair value reconciliation of Level 3 power derivative contract assets and liabilities measured at fair value on a recurring basis for the years ended December 31, 2025 and 2024:
| 2025 |  |  | 2024
| Ameren Missouri | Ameren Illinois | Ameren |  |  | Ameren Missouri | Ameren Illinois | Ameren
Beginning balance at January 1 | $ | 6 |  | $ | (53) |  | $ | (47) |  |  |  | $ | 4 |  | $ | (68) |  | $ | (64) |
Realized and unrealized gains (losses) included in regulatory assets/liabilities | 29 |  | (24) |  | 5 |  |  |  | 12 |  | (1) |  | 11 |
Settlements | (24) |  | 11 |  | (13) |  |  |  | (10) |  | 16 |  | 6 |
Ending balance at December 31 | $ | 11 |  | $ | (66) |  | $ | (55) |  |  |  | $ | 6 |  | $ | (53) |  | $ | (47) |
Change in unrealized gains (losses) related to assets/liabilities held at December 31 | 11 |  | (23) |  | (12) |  |  |  | 6 |  | 3 |  | 9 |
All gains or losses related to our Level 3 derivative commodity contracts are expected to be recovered or returned through customer rates; therefore, there is no impact to either net income or OCI resulting from changes in the fair value of these instruments. The following table describes the valuation techniques and significant unobservable inputs utilized for the fair value of our Level 3 power derivative contract assets and liabilities as of December 31, 2025 and 2024:
|  | Fair Value |  |  |  |  | Weighted Average(b)
| Commodity | Assets | Liabilities |  | Valuation Technique(s) | Unobservable Input(a) | Range
2025 | Power(c) | $ | 11 |  | $ | (66) |  |  | Discounted cash flow | Average forward peak and off-peak pricing – forwards/swaps ($/MWh) | 33 – 72 | 43
|  |  |  |  |  | Nodal basis ($/MWh) | (9) – (2) | (5)
2024 | Power(c) | $ | 6 |  | $ | (53) |  |  | Discounted cash flow | Average forward peak and off-peak pricing – forwards/swaps ($/MWh) | 32 – 69 | 45
|  |  |  |  |  | Nodal basis ($/MWh) | (8) – (2) | (5)
(a)Generally, significant increases (decreases) in these inputs in isolation would result in a significantly higher (lower) fair value measurement. (b)Unobservable inputs were weighted by relative fair value. (c)Valuations use visible forward prices adjusted for nodal-to-hub basis differentials. The following table sets forth the carrying amount and, by level within the fair value hierarchy, the fair value of long-term debt (including current portion), disclosed, but not recorded, at fair value as of December 31, 2025 and 2024:
| Carrying Amount(a) |  |  | Fair Value |  |
Long-Term Debt (Including Current Portion): |  |  |  | Level 2 |  | Level 3 |  | Total
| December 31, 2025
Ameren(b) | $ | 19,187 |  |  |  |  | $ | 17,433 |  |  | $ | 559 |  | (c) | $ | 17,992 |
Ameren Missouri(d) | 8,230 |  |  |  |  | 7,608 |  |  | — |  |  | 7,608 |
Ameren Illinois(d) | 6,259 |  |  |  |  | 5,753 |  |  | — |  |  | 5,753 |
| December 31, 2024
Ameren(b) | $ | 17,579 |  |  |  |  | $ | 15,395 |  |  | $ | 538 |  | (c) | $ | 15,933 |
Ameren Missouri(d) | 7,745 |  |  |  |  | 6,926 |  |  | — |  |  | 6,926 |
Ameren Illinois(d) | 5,852 |  |  |  |  | 5,243 |  |  | — |  |  | 5,243 |
(a)Included unamortized debt issuance costs, which were excluded from the fair value measurement, of $136 million, $62 million, and $56 million for Ameren, Ameren Missouri, and Ameren Illinois, respectively, as of December 31, 2025. Included unamortized debt issuance costs, which were excluded from the fair value measurement, of $129 million, $62 million, and $51 million for Ameren, Ameren Missouri, and Ameren Illinois, respectively, as of December 31, 2024. (b)Amount excludes Ameren (parent)’s repurchase of Ameren Missouri’s senior secured notes and first mortgage bonds and Ameren Illinois’ first mortgage bonds that were accounted for as a debt extinguishment. See Note 5 – Long-term Debt and Equity Financings for additional information. (c)The Level 3 fair value amount consists of ATXI’s senior unsecured notes. (d)Amount includes Ameren Missouri’s senior secured notes and first mortgage bonds and Ameren Illinois’ first mortgage bonds that were repurchased by Ameren (parent) in 2025 and 2024. The Ameren Companies’ carrying amounts of cash, cash equivalents, and restricted cash approximate fair value and are considered Level 1 in the fair value hierarchy. The Ameren Companies’ short-term borrowings approximate fair value because of the short-term nature of these instruments and are considered Level 2 in the fair value hierarchy.

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- Definition The entire disclosure for the fair value of financial instruments (as defined), including financial assets and financial liabilities (collectively, as defined), and the measurements of those instruments as well as disclosures related to the fair value of non-financial assets and liabilities. Such disclosures about the financial instruments, assets, and liabilities would include: (1) the fair value of the required items together with their carrying amounts (as appropriate); (2) for items for which it is not practicable to estimate fair value, disclosure would include: (a) information pertinent to estimating fair value (including, carrying amount, effective interest rate, and maturity, and (b) the reasons why it is not practicable to estimate fair value; (3) significant concentrations of credit risk including: (a) information about the activity, region, or economic characteristics identifying a concentration, (b) the maximum amount of loss the entity is exposed to based on the gross fair value of the related item, (c) policy for requiring collateral or other security and information as to accessing such collateral or security, and (d) the nature and brief description of such collateral or security; (4) quantitative information about market risks and how such risks are managed; (5) for items measured on both a recurring and nonrecurring basis information regarding the inputs used to develop the fair value measurement; and (6) for items presented in the financial statement for which fair value measurement is elected: (a) information necessary to understand the reasons for the election, (b) discussion of the effect of fair value changes on earnings, (c) a description of [similar groups] items for which the election is made and the relation thereof to the balance sheet, the aggregate carrying value of items included in the balance sheet that are not eligible for the election; (7) all other required (as defined) and desired information. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (bbb)(1) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482106/820-10-50-2 Reference 2: http://www.xbrl.org/2003/role/exampleRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 55 -Paragraph 107 -Publisher FASB -URI https://asc.fasb.org/1943274/2147482078/820-10-55-107 Reference 3: http://www.xbrl.org/2003/role/exampleRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 55 -Paragraph 100 -Publisher FASB -URI https://asc.fasb.org/1943274/2147482078/820-10-55-100 Reference 4: http://www.xbrl.org/2003/role/disclosureRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (c)(3) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482106/820-10-50-2 Reference 5: http://www.xbrl.org/2003/role/disclosureRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 6A -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482106/820-10-50-6A Reference 6: http://www.xbrl.org/2003/role/disclosureRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2E -Publisher FASB -URI https://asc.fasb.org/1943274/2147482106/820-10-50-2E Reference 7: http://www.xbrl.org/2003/role/disclosureRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 6A -Subparagraph (h) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482106/820-10-50-6A Reference 8: http://www.xbrl.org/2003/role/disclosureRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 6A -Subparagraph (f) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482106/820-10-50-6A Reference 9: http://www.xbrl.org/2003/role/disclosureRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 6A -Subparagraph (e) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482106/820-10-50-6A Reference 10: http://www.xbrl.org/2003/role/disclosureRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 6A -Subparagraph (d) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482106/820-10-50-6A Reference 11: http://www.xbrl.org/2003/role/disclosureRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (h) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482106/820-10-50-2 Reference 12: http://www.xbrl.org/2003/role/disclosureRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (g) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482106/820-10-50-2 Reference 13: http://www.xbrl.org/2003/role/disclosureRef -Topic 940 -SubTopic 820 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org/1943274/2147478119/940-820-50-1 + Details
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