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Long-Term Debt And Equity Financings (Narrative) (Details) - USD ($) $ / shares in Units, $ in Millions | 1 Months Ended | 3 Months Ended | 12 Months Ended |  |  |  |  |  |  |  |  |  |
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Jun. 30, 2025 | Mar. 31, 2025 | Dec. 31, 2024 | Nov. 30, 2024 | Sep. 30, 2025 | Dec. 31, 2025 | Dec. 31, 2024 | Dec. 31, 2023 | Apr. 30, 2025 | Oct. 31, 2024 | Sep. 30, 2024 | Aug. 31, 2024 | Jun. 30, 2024 | Apr. 30, 2024 | Jan. 31, 2024 | May 31, 2023 | May 31, 2022 | May 31, 2021
Long-Term Debt And Equity Financings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Gain (Loss) on Extinguishment of Debt |  |  |  |  |  | $ 8.0 | $ 16.0 | $ 0.0 |  |  |  |  |  |  |  |  |  |
Preferred stock, authorized (in shares) |  |  |  |  |  | 100,000,000 |  |  |  |  |  |  |  |  |  |  |  |
Preferred stock, par value (in dollars per share) |  |  |  |  |  | $ 0.01 |  |  |  |  |  |  |  |  |  |  |  |
Preferred stock, shares outstanding (in shares) |  |  |  |  |  | 0 |  |  |  |  |  |  |  |  |  |  |  |
Shares issued under the DRPlus and 401(k) plan |  |  |  |  |  | 400,000 | 500,000 | 600,000 |  |  |  |  |  |  |  |  |  |
Issuances of common stock |  |  |  |  |  | $ 574.0 | $ 273.0 | $ 346.0 |  |  |  |  |  |  |  |  |  |
Accrued Proceeds from Issuance of Common Stock |  |  | $ 7.0 |  |  | $ 7.0 | $ 7.0 |  |  |  |  |  |  |  |  |  |  |
Shares issued (in shares) |  |  |  |  |  | 300,000 | 200,000 | 500,000 |  |  |  |  |  |  |  |  |  |
Value of stock issued |  |  |  |  |  |  | $ 16.0 |  |  |  |  |  |  |  |  |  |  |
Common stock, shares authorized under DRPlus plan (in shares) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 3,000,000 |  |
Common stock, shares authorized under 401(k) Plan (in shares) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 7,500,000 |
Increase To Total Common Stock Available For Sale Under ATM Program |  |  |  |  | $ 1,250.0 |  |  |  |  |  |  |  |  |  |  |  |  |
Maximum value of shares to be issued under ATM Program |  |  |  |  |  | $ 1,500.0 |  |  |  |  |  |  |  |  |  |  |  | $ 3,000.0
Number of shares issued under ATM Program (in shares) |  |  |  |  |  | 5,800,000 | 2,900,000 | 3,200,000 |  |  |  |  |  |  |  |  |  |
Value of shares issued under ATM program |  |  |  |  |  | $ 530.0 | $ 233.0 | $ 299.0 |  |  |  |  |  |  |  |  |  |
Payments of stock issuance costs |  |  |  |  |  | 5.0 | 2.0 | 3.0 |  |  |  |  |  |  |  |  |  |
Debt default provision excess |  |  |  |  |  | $ 25.0 |  |  |  |  |  |  |  |  |  |  |  |
Forward Sale Agreements Outstanding |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Long-Term Debt And Equity Financings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of shares to settle forward sale agreements (in shares) |  |  |  |  |  | 6,400,000 |  |  |  |  |  |  |  |  |  |  |  |
Cash settlement of forward sale agreements |  |  |  |  |  | $ 585.0 |  |  |  |  |  |  |  |  |  |  |  |
Period End Net Cash Settlement Price |  |  |  |  |  | 52.0 |  |  |  |  |  |  |  |  |  |  |  |
Period end net share settlement price |  |  |  |  |  | 0.5 |  |  |  |  |  |  |  |  |  |  |  |
Forward sale agreement equity offering shares (in shares) |  |  |  |  |  | 6.4 |  |  |  |  |  |  |  |  |  |  |  |
Forward Sale Agreement Gross Sales Price |  |  |  |  |  | 600.0 |  |  |  |  |  |  |  |  |  |  |  |
Dividend reinvestment and 401 (k) plans |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Long-Term Debt And Equity Financings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Issuances of common stock |  |  |  |  |  | $ 37.0 | 33.0 | $ 39.0 |  |  |  |  |  |  |  |  |  |
Ameren Missouri |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Long-Term Debt And Equity Financings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Preferred stock, authorized (in shares) |  |  |  |  |  | 7,500,000 |  |  |  |  |  |  |  |  |  |  |  |
Preferred stock, par value (in dollars per share) |  |  |  |  |  | $ 1 |  |  |  |  |  |  |  |  |  |  |  |
Preferred stock, shares outstanding (in shares) |  |  |  |  |  | 0 |  |  |  |  |  |  |  |  |  |  |  |
Ameren Missouri | Secured Debt | Related Party |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Long-Term Debt And Equity Financings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt instrument face amount |  |  | 58.0 |  |  | $ 88.0 | 58.0 |  |  |  |  |  |  |  |  |  |  |
Ameren Illinois |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Long-Term Debt And Equity Financings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Preferred stock, authorized (in shares) |  |  |  |  |  | 2,600,000 |  |  |  |  |  |  |  |  |  |  |  |
Preferred stock, par value (in dollars per share) |  |  |  |  |  | $ 0 |  |  |  |  |  |  |  |  |  |  |  |
Preferred stock, shares outstanding (in shares) |  |  |  |  |  | 0 |  |  |  |  |  |  |  |  |  |  |  |
Common stock equity to capitalization ratio |  |  |  |  |  | 55.00% |  |  |  |  |  |  |  |  |  |  |  |
Ameren Illinois | Secured Debt | Related Party |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Long-Term Debt And Equity Financings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt instrument face amount |  |  | 3.0 |  |  | $ 5.0 | 3.0 |  |  |  |  |  |  |  |  |  |  |
Ameren Missouri and Ameren Illinois |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Long-Term Debt And Equity Financings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Bonds interest rate assumption |  |  |  |  |  | 7.00% |  |  |  |  |  |  |  |  |  |  |  |
Dividend rate on preferred shares, percentage |  |  |  |  |  | 8.00% |  |  |  |  |  |  |  |  |  |  |  |
Senior Secured Notes and First Mortgage Bonds | Secured Debt | Related Party |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Long-Term Debt And Equity Financings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Gain (Loss) on Extinguishment of Debt |  |  |  |  |  | $ 8.0 | 16.0 |  |  |  |  |  |  |  |  |  |  |
Interest Expense, Debt |  |  |  |  |  | 3.0 | 1.0 |  |  |  |  |  |  |  |  |  |  |
Senior Secured Notes and First Mortgage Bonds | Ameren (parent) | Secured Debt | Related Party |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Long-Term Debt And Equity Financings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Repayments of Senior Secured Notes and First Mortgage Bonds | $ 24.0 |  | 44.0 | $ 44.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Senior Unsecured Notes, 5.375%, Due 2035 | Ameren (parent) | Unsecured Debt |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Long-Term Debt And Equity Financings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt instrument face amount |  |  | 0.0 |  |  | $ 750.0 | 0.0 |  |  |  |  |  |  |  |  |  |  |
Long-term debt interest rate |  |  |  |  |  | 5.375% |  |  |  |  |  |  |  |  |  |  |  |
Senior Unsecured Notes, 5.375%, Due 2035 | Ameren Missouri | Secured Debt |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Long-Term Debt And Equity Financings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt instrument face amount |  | $ 750.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Long-term debt interest rate |  | 5.375% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
2.50% Senior unsecured notes due 2024 | Ameren Missouri | Secured Debt |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Long-Term Debt And Equity Financings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt instrument face amount |  |  |  |  |  |  |  |  |  |  | $ 450.0 |  |  |  |  |  |  |
Long-term debt interest rate |  |  |  |  |  |  |  |  |  |  | 2.50% |  |  |  |  |  |  |
First Mortgage Bonds, 5.25%, Due 2035 | Ameren Missouri | Secured Debt |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Long-Term Debt And Equity Financings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt instrument face amount |  |  | 0.0 |  |  | $ 500.0 | 0.0 |  | $ 500.0 |  |  |  |  |  |  |  |  |
Long-term debt interest rate |  |  |  |  |  | 5.25% |  |  | 5.25% |  |  |  |  |  |  |  |  |
First Mortgage Bonds, 5.25%, Due 2054 - $350 Issuance | Ameren Missouri | Secured Debt |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Long-Term Debt And Equity Financings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt instrument face amount |  |  | 350.0 |  |  | $ 350.0 | 350.0 |  |  |  |  |  |  |  | $ 350.0 |  |  |
Long-term debt interest rate |  |  |  |  |  | 5.25% |  |  |  |  |  |  |  |  | 5.25% |  |  |
Senior Secured Notes 5.20% Due 2034 | Ameren Missouri | Secured Debt |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Long-Term Debt And Equity Financings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt instrument face amount |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 500.0 |  |  |  |
Long-term debt interest rate |  |  |  |  |  |  |  |  |  |  |  |  |  | 5.20% |  |  |  |
3.50% senior secured notes due 2024 | Ameren Missouri | Secured Debt |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Long-Term Debt And Equity Financings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt instrument face amount |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 350.0 |  |  |  |
Long-term debt interest rate |  |  |  |  |  |  |  |  |  |  |  |  |  | 3.50% |  |  |  |
First Mortgage Bonds, 5.125%, Due 2055 - $450 Issuance | Ameren Missouri | Secured Debt |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Long-Term Debt And Equity Financings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt instrument face amount |  |  | 450.0 |  |  | $ 450.0 | 450.0 |  |  | $ 450.0 |  |  |  |  |  |  |  |
Long-term debt interest rate |  |  |  |  |  | 5.125% |  |  |  | 5.125% |  |  |  |  |  |  |  |
4.85% Securitized Utility Tariff Bonds Due 2039 | Ameren Missouri | Secured Debt |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Long-Term Debt And Equity Financings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt instrument face amount |  |  | $ 476.0 |  |  | $ 459.0 | $ 476.0 |  |  |  |  |  |  |  |  |  |  |
Long-term debt interest rate |  |  | 4.85% |  |  | 4.85% | 4.85% |  |  |  |  |  |  |  |  |  |  |
First Mortgage Bonds, 5.625%, Due 2055 | Ameren Illinois | Secured Debt |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Long-Term Debt And Equity Financings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt instrument face amount | $ 350.0 | $ 350.0 | $ 0.0 |  |  | $ 700.0 | $ 0.0 |  |  |  |  |  |  |  |  |  |  |
Long-term debt interest rate |  | 5.625% |  |  | 5.625% | 5.625% |  |  |  |  |  |  |  |  |  |  |  |
Senior Secured Notes 3.25% Due 2025 | Ameren Illinois | Secured Debt |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Long-Term Debt And Equity Financings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Long-term debt interest rate |  | 3.25% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Repayments of Debt |  | $ 300.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
5.55% First mortgage bonds due 2054 | Ameren Illinois | Secured Debt |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Long-Term Debt And Equity Financings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt instrument face amount |  |  | 625.0 |  |  | $ 625.0 | 625.0 |  |  |  |  |  | $ 625.0 |  |  |  |  |
Long-term debt interest rate |  |  |  |  |  | 5.55% |  |  |  |  |  |  | 5.55% |  |  |  |  |
Senior Unsecured Notes, 5.17%, Due 2039 | Ameren Transmission Company of Illinois | Unsecured Debt |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Long-Term Debt And Equity Financings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt instrument face amount |  |  | 70.0 |  |  | $ 70.0 | 70.0 |  |  |  |  | $ 70.0 |  |  |  |  |  |
Long-term debt interest rate |  |  |  |  |  | 5.17% |  |  |  |  |  | 5.17% |  |  |  |  |  |
Senior Unsecured Notes, 5.42%, Due 2053 | Ameren Transmission Company of Illinois | Unsecured Debt |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Long-Term Debt And Equity Financings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt instrument face amount |  |  | 70.0 |  |  | $ 70.0 | 70.0 |  |  |  |  | $ 70.0 |  |  |  |  |  |
Long-term debt interest rate |  |  |  |  |  | 5.42% |  |  |  |  |  | 5.42% |  |  |  |  |  |
3.43% Senior unsecured notes due 2050 | Ameren Transmission Company of Illinois | Unsecured Debt |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Long-Term Debt And Equity Financings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt instrument face amount |  |  | $ 351.0 |  |  | $ 351.0 | $ 351.0 |  |  |  |  |  |  |  |  |  |  |
Long-term debt interest rate |  |  |  |  |  | 3.43% |  |  |  |  |  | 3.43% |  |  |  |  |  |
Long-Term Debt, Maturity, Remainder of Fiscal Year |  |  |  |  |  |  |  |  |  |  |  | $ 49.0 |  |  |  |  |  |
Ratio of Indebtedness to Net Capital |  |  |  |  |  | 0.70 |  |  |  |  |  |  |  |  |  |  |  |
Ratio of Indebtedness to Total Assets |  |  |  |  |  | 0.10 |  |  |  |  |  |  |  |  |  |  |  |
Minimum |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Long-Term Debt And Equity Financings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Forward contract indexed to issuer's equity, forward rate per share (in dollars per share) |  |  |  |  |  | $ 91.89 |  |  |  |  |  |  |  |  |  |  |  |
Minimum | Ameren Illinois |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Long-Term Debt And Equity Financings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Common stock equity to capitalization ratio |  |  |  |  |  | 30.00% |  |  |  |  |  |  |  |  |  |  |  |

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- Definition Additional Shares of Ameren common stock to be issued under its 401(k) plan pursuant to filing a Form S-8 registration statement with the SEC. + References No definition available. + Details
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- Definition Additional Shares of Ameren common stock to be issued under its DRPlus plan pursuant to filing a Form S-3 registration statement with the SEC. + References No definition available. + Details
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