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Retirement Benefits | 9 Months Ended
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Sep. 30, 2025
Retirement Benefits [Abstract] |
RETIREMENT BENEFITS | RETIREMENT BENEFITS The following table presents the components of the net periodic benefit cost (income) incurred for Ameren’s pension and postretirement benefit plans for the three and nine months ended September 30, 2025 and 2024:
| Pension Benefits |  |  | Postretirement Benefits
| Three Months |  | Nine Months |  |  | Three Months |  | Nine Months
| 2025 |  | 2024 |  | 2025 |  | 2024 |  |  | 2025 |  | 2024 |  | 2025 |  | 2024
Service cost(a) | $ | 21 |  |  | $ | 22 |  |  | $ | 62 |  |  | $ | 66 |  |  |  | $ | 3 |  |  | $ | 3 |  |  | $ | 8 |  |  | $ | 9 |
Non-service cost components: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Interest cost | 58 |  |  | 56 |  |  | 175 |  |  | 167 |  |  |  | 11 |  |  | 11 |  |  | 33 |  |  | 33 |
Expected return on plan assets(b) | (76) |  |  | (82) |  |  | (227) |  |  | (246) |  |  |  | (24) |  |  | (23) |  |  | (71) |  |  | (69) |
Amortization of(b): |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Prior service cost (credit) | — |  |  | — |  |  | — |  |  | — |  |  |  | (1) |  |  | (1) |  |  | (3) |  |  | (3) |
Actuarial (gain) | (9) |  |  | (17) |  |  | (26) |  |  | (50) |  |  |  | (9) |  |  | (9) |  |  | (27) |  |  | (29) |
Total non-service cost components(c) | $ | (27) |  |  | $ | (43) |  |  | $ | (78) |  |  | $ | (129) |  |  |  | $ | (23) |  |  | $ | (22) |  |  | $ | (68) |  |  | $ | (68) |
Net periodic benefit income(d) | $ | (6) |  |  | $ | (21) |  |  | $ | (16) |  |  | $ | (63) |  |  |  | $ | (20) |  |  | $ | (19) |  |  | $ | (60) |  |  | $ | (59) |
(a)Service cost, net of capitalization, is reflected in “Operating Expenses – Other operations and maintenance” on Ameren’s statement of income. (b)Prior service cost (credit) is amortized on a straight-line basis over the average future service of active participants benefiting under a plan amendment. Net actuarial gains or losses related to the net benefit obligation subject to amortization are amortized on a straight-line basis over 10 years. The difference between the actual and expected return on plan assets is amortized over 4 years. (c)Non-service cost components are reflected in “Other Income, Net” on Ameren’s consolidated statement of income. See Note 5 – Other Income, Net for additional information. (d)Does not include the impact of the tracker for the difference between the level of pension and postretirement benefit costs (income) incurred by Ameren Missouri under GAAP and the level of such costs included in customer rates. Ameren Missouri and Ameren Illinois are responsible for their respective share of Ameren’s pension and other postretirement costs. The following table presents the respective share of net periodic pension and other postretirement benefit costs (income) incurred for the three and nine months ended September 30, 2025 and 2024:
| Pension Benefits |  |  | Postretirement Benefits
| Three Months |  | Nine Months |  |  | Three Months |  | Nine Months
| 2025 |  | 2024 |  | 2025 |  | 2024 |  |  | 2025 |  | 2024 |  | 2025 |  | 2024
Ameren Missouri(a) | $ | (4) |  |  | $ | (11) |  |  | $ | (11) |  |  | $ | (33) |  |  |  | $ | (7) |  |  | $ | (6) |  |  | $ | (21) |  |  | $ | (20) |
Ameren Illinois | (1) |  |  | (8) |  |  | (3) |  |  | (25) |  |  |  | (13) |  |  | (13) |  |  | (39) |  |  | (39) |
Other | (1) |  |  | (2) |  |  | (2) |  |  | (5) |  |  |  | — |  |  | — |  |  | — |  |  | — |
Ameren(a) | $ | (6) |  |  | $ | (21) |  |  | $ | (16) |  |  | $ | (63) |  |  |  | $ | (20) |  |  | $ | (19) |  |  | $ | (60) |  |  | $ | (59) |
(a)Does not include the impact of the tracker for the difference between the level of pension and postretirement benefit costs (income) incurred by Ameren Missouri under GAAP and the level of such costs included in customer rates.

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