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Face (par) amount of debt instrument at time of issuance.

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Contractual interest rate for funds borrowed, under the debt agreement.

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The per share price of the Company's stock at which the contract holder of the freestanding contract has the right to purchase or sell the Company's stock at a future date.

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The number of issuer's shares to which the forward contract is indexed.

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Amount that would be paid upon settlement of forward contract indexed to equity.

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Number of shares as a settlement alternative for each freestanding forward contract.

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Difference between the fair value of payments made and the carrying amount of debt which is extinguished prior to maturity.

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Amount of the cost of borrowed funds accounted for as interest expense for debt.

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The cash inflow from the additional capital contribution to the entity.

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For a forward contract or an option indexed to the issuer's equity shares, the number of issuer's shares to which the contract is indexed.

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Number of shares issued during the period from a dividend reinvestment plan (DRIP). A dividend reinvestment plan allows the shareholders to reinvest dividends paid to them by the entity on new issues of stock by the entity.

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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