Stock-Based Compensation (Summary of Expense) (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation expense	$ 28	$ 28	$ 26
Employee service share-based compensation, tax benefit from compensation expense	7	7	7
Share-based Compensation Expense, Net of Tax	21	21	19
Ameren Missouri
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation expense	8	8	6
Ameren Illinois
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation expense	4	4	4
Other
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation expense	$ 16	$ 16	$ 16
Performance Share Units \| Market Condition PSUs
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Fair value of share units awarded	$ 123.19
Performance Share Units \| Market Condition PSUs February Grant
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Fair value of share units awarded	$ 123.19	$ 56.73	$ 91.07
Three-year risk-free rate	4.23%	4.25%	4.19%
Ameren’s common stock volatility	21.00%	21.00%	26.00%
Volatility range for peer group, minimum	19.00%	19.00%	24.00%
Volatility range for peer group, maximum	24.00%	23.00%	32.00%

Amount of expense for award under share-based payment arrangement. Excludes amount capitalized.

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Amount, after tax, of expense for award under share-based payment arrangement.

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The weighted average fair value at grant date for nonvested equity-based awards issued during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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The estimated measure of the percentage by which a share price is expected to fluctuate during a period. Volatility also may be defined as a probability-weighted measure of the dispersion of returns about the mean. The volatility of a share price is the standard deviation of the continuously compounded rates of return on the share over a specified period. That is the same as the standard deviation of the differences in the natural logarithms of the stock prices plus dividends, if any, over the period.

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The risk-free interest rate assumption that is used in valuing an option on its own shares.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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