Stock-Based Compensation (Summary of Expense) (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation expense	$ 21	$ 20	$ 20
Employee service share-based compensation, tax benefit from compensation expense	6	5	6
Share-based Compensation Expense, Net of Tax	15	15	14
Ameren Missouri [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation expense	5	4	4
Ameren Illinois Company
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation expense	3	3	3
Other
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation expense	$ 13	$ 13	$ 13
Performance Share Units \| Market Condition PSUs
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Fair value of share units awarded	$ 82.49	$ 67.42	$ 62.88
Three-year risk-free rate	1.62%	2.46%	1.98%
Ameren’s common stock volatility	15.00%	17.00%	17.00%
Volatility range for peer group, minimum	14.00%	15.00%	15.00%
Volatility range for peer group, maximum	28.00%	25.00%	23.00%

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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