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Long-Term debt and Equity Financings (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions | 3 Months Ended | 6 Months Ended | 9 Months Ended
--- | --- | --- | ---
Sep. 30, 2025 | Sep. 30, 2024 | Jun. 30, 2025 | Sep. 30, 2025 | Sep. 30, 2024
Long-Term Debt And Equity Financings [Line Items] |  |  |  |  |
Shares issued under the DRPlus and 401(k) plan | 100 | 100 |  | 300 | 400
Issuances of common stock |  |  |  | $ 35.0 | $ 30.0
Stock Issued During Period, Shares, Other (in shares) | 0 | 0 | 300 | 300 | 200
Stock Issued During Period, Value, Other | $ 25.0 |  |  |  |
Maximum Value Of Shares To Be Issued Under ATM Program | $ 1,500.0 |  |  | $ 1,500.0 |
Financial Instruments Subject to Mandatory Redemption, Settlement Terms, Number of Shares Indexed | 5,800 |  |  | 5,800 |
Forward Contract Indexed to Issuer's Equity, Forward Rate Per Share |  |  |  | $ 91.89 |
Forward Contract Indexed to Issuer's Equity, Indexed Shares | 6,400 |  |  | 6,400 |
Gain (Loss) on Extinguishment of Debt | $ 0.0 | $ 0.0 |  | $ 8.0 | $ 0.0
Total Common Stock Available For Sale Under ATM Program | 3,000.0 |  |  | 3,000.0 |
Increase To Total Common Stock Available For Sale Under ATM Program | 1,250.0 |  |  |  |
Dividend reinvestment and 401 (k) plans |  |  |  |  |
Long-Term Debt And Equity Financings [Line Items] |  |  |  |  |
Issuances of common stock | 3.0 |  |  | 28.0 |
Accrued Proceeds from Issuance of Common Stock |  |  |  | 7.0 |
Senior unsecured Notes 5.375% Due 2035 | Unsecured Debt | Ameren (parent) |  |  |  |  |
Long-Term Debt And Equity Financings [Line Items] |  |  |  |  |
Debt Instrument, Face Amount | $ 750.0 |  |  | $ 750.0 |
Debt Instrument, Interest Rate, Stated Percentage | 5.375% |  |  | 5.375% |
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 |
Interest Expense, Debt | $ 1.0 |  |  | 2.0 |
Senior Secured Notes and First Mortgage Bonds | Secured Debt | Ameren (parent) | Related Party |  |  |  |  |
Long-Term Debt And Equity Financings [Line Items] |  |  |  |  |
Repayments of Senior Secured Notes and First Mortgage Bonds |  |  |  | 24.0 |
First Mortgage Bonds, 5.25%, Due 2035 - $500 Issuance | Secured Debt | Union Electric Company |  |  |  |  |
Long-Term Debt And Equity Financings [Line Items] |  |  |  |  |
Debt Instrument, Face Amount | $ 500.0 |  |  | $ 500.0 |
Debt Instrument, Interest Rate, Stated Percentage | 5.25% |  |  | 5.25% |
First Mortgage Bonds, 5.625%, Due 2055 - $350 March Issuance | Secured Debt | Ameren Illinois Company |  |  |  |  |
Long-Term Debt And Equity Financings [Line Items] |  |  |  |  |
Debt Instrument, Face Amount | $ 350.0 |  |  | $ 350.0 |
Debt Instrument, Interest Rate, Stated Percentage | 5.625% |  |  | 5.625% |
First Mortgage Bonds, 5.625%, Due 2055 - $350 September Issuance | Secured Debt | Ameren Illinois Company |  |  |  |  |
Long-Term Debt And Equity Financings [Line Items] |  |  |  |  |
Debt Instrument, Face Amount | $ 350.0 |  |  | $ 350.0 |
Senior Secured Notes, 3.25%, Due 2025 | Secured Debt | Ameren Illinois Company |  |  |  |  |
Long-Term Debt And Equity Financings [Line Items] |  |  |  |  |
Debt Instrument, Face Amount | $ 300.0 |  |  | $ 300.0 |
Debt Instrument, Interest Rate, Stated Percentage | 3.25% |  |  | 3.25% |
Gain (Loss) on Extinguishment of Debt |  |  |  | $ 300.0 |
Forward Sale Agreements Outstanding |  |  |  |  |
Long-Term Debt And Equity Financings [Line Items] |  |  |  |  |
Forward Contract Indexed to Issuer's Equity, Settlement Alternatives, Shares, at Fair Value | 12,200 |  |  | 12,200 |
Forward Contract Indexed to Issuer's Equity, Settlement Alternatives, Cash, at Fair Value | $ 1,100.0 |  |  | $ 1,100.0 |
Period End Net Cash Settlement Price | 158.0 |  |  | 158.0 |
Period End Net Share Settlement Price | $ 1.5 |  |  | $ 1.5 |
Forward Sale Agreement Equity Offering Shares | 12,200 |  |  | 12,200 |
Forward Sale Agreement Gross Sales Price | $ 1,100.0 |  |  | $ 1,100.0 |
Minimum |  |  |  |  |
Long-Term Debt And Equity Financings [Line Items] |  |  |  |  |
Forward Contract Indexed to Issuer's Equity, Forward Rate Per Share |  |  |  | $ 81.00 |
Maximum |  |  |  |  |
Long-Term Debt And Equity Financings [Line Items] |  |  |  |  |
Forward Contract Indexed to Issuer's Equity, Forward Rate Per Share |  |  |  | 98.66 |
Weighted Average |  |  |  |  |
Long-Term Debt And Equity Financings [Line Items] |  |  |  |  |
Forward Contract Indexed to Issuer's Equity, Forward Rate Per Share |  |  |  | $ 91.02 |

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