Stock-Based Compensation (Summary of Expense) (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation expense	$ 26	$ 24	$ 22
Employee service share-based compensation, tax benefit from compensation expense	7	6	6
Share-based Compensation Expense, Net of Tax	19	18	16
Ameren Missouri
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation expense	6	4	5
Ameren Illinois
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation expense	4	2	3
Other
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation expense	$ 16	$ 18	$ 14
Performance Share Units \| Market Condition PSUs
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Fair value of share units awarded	$ 88.52
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	$ 91.07	$ 92.75	$ 87.11
Three-year risk-free rate	4.19%	1.80%	0.17%
Ameren’s common stock volatility	26.00%	29.00%	28.00%
Volatility range for peer group, minimum	24.00%	26.00%	26.00%
Volatility range for peer group, maximum	32.00%	35.00%	36.00%

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

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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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