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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%

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- Definition 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. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 718 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (f)(2)(ii) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480429/718-10-50-2 + Details
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- Definition The risk-free interest rate assumption that is used in valuing an option on its own shares. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 718 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (f)(2)(iv) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480429/718-10-50-2 + Details
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