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Stock-Based Compensation (Tables) | 12 Months Ended
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Dec. 31, 2023
Share-Based Payment Arrangement [Abstract] |
Summary Of Nonvested Shares Related To Long-Term Incentive Plan | The following table summarizes Ameren’s outstanding performance share unit and restricted stock unit activity for the year ended December 31, 2023:
| Performance Share Units – Market Condition(a) |  | Performance Share Units – Performance Condition(b) |  | Restricted Stock Units
| Share Units |  | Weighted-average Fair Value per Share Unit |  | Share Units |  | Weighted-average Fair Value per Share Unit |  | Stock Units |  | Weighted-average Fair Value per Stock Unit
Outstanding at January 1, 2023(c) | 744,273 |  |  | $ | 87.23 |  |  | 119,737 |  |  | $ | 80.65 |  |  | 436,812 |  |  | $ | 80.94 |
Granted | 311,674 |  |  | 88.52 |  |  | 40,118 |  |  | 84.70 |  |  | 130,600 |  |  | 84.44 |
Forfeitures | (45,080) |  |  | 90.97 |  |  | (7,389) |  |  | 84.73 |  |  | (22,566) |  |  | 84.75 |
Dividend equivalents(d) | 14,749 |  |  | 89.79 |  |  | 2,372 |  |  | 82.66 |  |  | 9,132 |  |  | 82.44 |
Vested and distributed | (263,904) |  |  | 82.50 |  |  | (42,447) |  |  | 76.70 |  |  | (176,114) |  |  | 76.94 |
Outstanding at December 31, 2023(c) | 761,712 |  |  | $ | 89.22 |  |  | 112,391 |  |  | $ | 83.36 |  |  | 377,864 |  |  | $ | 83.82 |
(a)The exact number of shares issued pursuant to a share unit varies from 0% to 200% of the target award, depending on actual company performance relative to the specified market conditions. Compensation cost on nonforfeited awards is recognized regardless of whether Ameren achieves the specified market conditions. (b)The exact number of shares issued pursuant to a share unit varies from 0% to 200% of the target award, depending on actual company performance relative to the performance goals. Compensation cost is recognized ratably over the requisite service period only for awards for which it is probable that the performance condition will be satisfied. (c)Outstanding awards include awards that vest on a pro-rata basis due to attainment of retirement eligibility by certain employees, but have not yet been distributed. In these cases, the pro-rata basis awards have not yet been distributed as the entire performance period has not been completed. The number of shares issued for retirement-eligible employees will vary depending on actual performance over the three-year performance period. (d)Dividend equivalents represent the right to receive shares measured by the dividend payable with respect to the corresponding number of outstanding share units. Dividend equivalents will accrue and be reinvested in additional share units throughout the performance period.
Disclosure of Share-based Compensation Arrangements by Share-based Payment Award | The following table presents the fair value of each share unit along with the significant assumptions used to calculate the fair value of each share unit for the years ended December 31, 2023, 2022, and 2021:
| 2023 | 2022 | 2021
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(a) | 26% | 29% | 28%
Volatility range for the peer group(a) | 24% – 32% | 26% – 35% | 26% – 36%
(a)Based on a historical period that is equal to the remaining term of the performance period as of the grant date. The following table presents the stock-based compensation expense for the years ended December 31, 2023, 2022, and 2021:
| 2023 |  | 2022 |  | 2021
Ameren Missouri | $ | 6 |  |  | $ | 4 |  |  | $ | 5 |
Ameren Illinois | 4 |  |  | 2 |  |  | 3 |
Other(a) | 16 |  |  | 18 |  |  | 14 |
Ameren | 26 |  |  | 24 |  |  | 22 |
Less: Income tax benefit | 7 |  |  | 6 |  |  | 6 |
Stock-based compensation expense, net | $ | 19 |  |  | $ | 18 |  |  | $ | 16 |
(a)Represents compensation expense for employees of Ameren Services. These amounts are not included in the Ameren Missouri and Ameren Illinois amounts above.

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