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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
	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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