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Retirement Benefit Obligations | 9 Months Ended
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Sep. 30, 2025
Retirement Benefits [Abstract] |
Retirement Benefit Obligations | Note 9: Retirement Benefit obligations The Company co-sponsors the Unitil Corporation Retirement Plan (Pension Plan), the Unitil Retiree Health and Welfare Benefits Plan (PBOP Plan), and the Unitil Corporation SERP to provide certain pension and postretirement benefits for its retirees and current employees. Please see Note 9 to the Consolidated Financial Statements in the Company’s Form 10-K for the year ended December 31, 2024 as filed with the SEC on February 10, 2025 for additional information regarding these plans. The following table includes the key weighted average assumptions used in determining the Company’s benefit plan costs and obligations:
Used to Determine Plan Costs |  | 2025 |  |  | 2024 |
Discount Rate |  |  | 5.60 | % |  |  | 5.00 | %
Rate of Compensation Increase |  |  | 3.00 | % |  |  | 3.00 | %
Expected Long-term rate of return on plan assets |  |  | 7.50 | % |  |  | 7.50 | %
The health care cost trend rate used to determine benefit plan costs for 2025 for pre-65 retirees is 8.50%, with an ultimate rate of 4.50% in 2034, and for post-65 retirees, the health care cost trend rate is 7.50%, with an ultimate rate of 4.50% in 2034. The health care cost trend rate used to determine benefit plan costs for 2024 for pre-65 retirees was 8.00%, with an ultimate rate of 4.50% in 2033, and for post-65 retirees, the health care cost trend rate was 6.00%, with an ultimate rate of 4.50% in 2033. The following tables provide the components of the Company’s Retirement plan costs ($000’s):
|  | Pension Plan |  |  | PBOP Plan |  |  | SERP |
For the Three Months Ended September 30, |  | 2025 |  |  | 2024 |  |  | 2025 |  |  | 2024 |  |  | 2025 |  |  | 2024 |
Service Cost |  | $ | 436 |  |  | $ | 498 |  |  | $ | 502 |  |  | $ | 497 |  |  | $ | 39 |  |  | $ | 53 |
Interest Cost |  |  | 2,016 |  |  |  | 1,887 |  |  |  | 992 |  |  |  | 817 |  |  |  | 172 |  |  |  | 180 |
Expected Return on Plan Assets |  |  | (2,624 | ) |  |  | (2,649 | ) |  |  | (1,086 | ) |  |  | (970 | ) |  |  | — |  |  |  | — |
Prior Service Cost Amortization |  |  | 63 |  |  |  | 80 |  |  |  | — |  |  |  | — |  |  |  | 3 |  |  |  | 4 |
Actuarial Loss Amortization |  |  | 320 |  |  |  | 361 |  |  |  | (187 | ) |  |  | (218 | ) |  |  | (54 | ) |  |  | — |
Sub-total |  |  | 211 |  |  |  | 177 |  |  |  | 221 |  |  |  | 126 |  |  |  | 160 |  |  |  | 237 |
Amounts Capitalized and Deferred |  |  | 6 |  |  |  | 30 |  |  |  | (64 | ) |  |  | (8 | ) |  |  | (51 | ) |  |  | (73 | )
Net Periodic Benefit Cost Recognized |  | $ | 217 |  |  | $ | 207 |  |  | $ | 157 |  |  | $ | 118 |  |  | $ | 109 |  |  | $ | 164 |
|  | Pension Plan |  |  | PBOP Plan |  |  | SERP |
For the Nine Months Ended September 30, |  | 2025 |  |  | 2024 |  |  | 2025 |  |  | 2024 |  |  | 2025 |  |  | 2024 |
Service Cost |  | $ | 1,308 |  |  | $ | 1,492 |  |  | $ | 1,505 |  |  | $ | 1,491 |  |  | $ | 118 |  |  | $ | 162 |
Interest Cost |  |  | 6,048 |  |  |  | 5,659 |  |  |  | 2,976 |  |  |  | 2,451 |  |  |  | 516 |  |  |  | 542 |
Expected Return on Plan Assets |  |  | (7,873 | ) |  |  | (7,947 | ) |  |  | (3,257 | ) |  |  | (2,908 | ) |  |  | — |  |  |  | — |
Prior Service Cost Amortization |  |  | 191 |  |  |  | 242 |  |  |  | — |  |  |  | — |  |  |  | 7 |  |  |  | 8 |
Actuarial Loss Amortization |  |  | 958 |  |  |  | 1,085 |  |  |  | (561 | ) |  |  | (655 | ) |  |  | (161 | ) |  |  | — |
Sub-total |  |  | 632 |  |  |  | 531 |  |  |  | 663 |  |  |  | 379 |  |  |  | 480 |  |  |  | 712 |
Amounts Capitalized and Deferred |  |  | 141 |  |  |  | 165 |  |  |  | (98 | ) |  |  | 170 |  |  |  | (152 | ) |  |  | (221 | )
Net Periodic Benefit Cost Recognized |  | $ | 773 |  |  | $ | 696 |  |  | $ | 565 |  |  | $ | 549 |  |  | $ | 328 |  |  | $ | 491 |
Employer Contributions As of September 30, 2025, the Company had made contributions of $3.9 million and $0.6 million to its Pension Plan and PBOP Plan, respectively, in 2025. The Company, along with its subsidiaries, expects to make contributions to its Pension and PBOP Plans in 2025 and future years at minimum required and discretionary funding levels consistent with the amounts recovered in the distribution utilities’ rates for these Pension and PBOP Plan costs. As of September 30, 2025, the Company had made $0.5 million of benefit payments under the SERP Plan in 2025. The Company presently anticipates making an additional $0.2 million of benefit payments under the SERP Plan in 2025.

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