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Retirement Benefit obligations | 3 Months Ended
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Mar. 31, 2024
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, 2023 as filed with the SEC on February 13, 2024 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 |  | 2024 |  |  | 2023 |
Discount Rate |  |  | 5.00 | % |  |  | 5.25 | %
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 2024 for pre-65 retirees is 8.00%, with an ultimate rate of 4.50% in 2033, and for post-65 retirees, the health care cost trend rate is 6.00%, with an ultimate rate of 4.50% in 2033. The health care cost trend rate used to determine benefit plan costs for 2023 for pre-65 retirees was 8.00%, with an ultimate rate of 4.50% in 2030, and for post-65 retirees, the health care cost trend rate was 6.25%, with an ultimate rate of 4.50% in 2030. 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 March 31, |  | 2024 |  |  | 2023 |  |  | 2024 |  |  | 2023 |  |  | 2024 |  |  | 2023 |
Service Cost |  | $ | 497 |  |  | $ | 523 |  |  | $ | 497 |  |  | $ | 373 |  |  | $ | 54 |  |  | $ | 63 |
Interest Cost |  |  | 1,886 |  |  |  | 1,870 |  |  |  | 817 |  |  |  | 725 |  |  |  | 181 |  |  |  | 188 |
Expected Return on Plan Assets |  |  | (2,649 | ) |  |  | (2,672 | ) |  |  | (969 | ) |  |  | (852 | ) |  |  | — |  |  |  | — |
Prior Service Cost Amortization |  |  | 81 |  |  |  | 89 |  |  |  | — |  |  |  | 198 |  |  |  | 2 |  |  |  | 14 |
Actuarial Loss Amortization |  |  | 362 |  |  |  | — |  |  |  | (219 | ) |  |  | (366 | ) |  |  | — |  |  |  | 0 |
Sub-total |  |  | 177 |  |  |  | (190 | ) |  |  | 126 |  |  |  | 78 |  |  |  | 237 |  |  |  | 265 |
Amounts Capitalized and Deferred |  |  | 108 |  |  |  | 498 |  |  |  | 124 |  |  |  | 221 |  |  |  | (73 | ) |  |  | (82 | )
Net Periodic Benefit Cost Recognized |  | $ | 285 |  |  | $ | 308 |  |  | $ | 250 |  |  | $ | 299 |  |  | $ | 164 |  |  | $ | 183 |
Employer Contributions As of March 31, 2024, the Company had no t made any contributions to its Pension Plan and PBOP Plan, respectively, in 2024. The Company, along with its subsidiaries, expects to make contributions to its Pension and PBOP Plans in 2024 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 March 31, 2024, the Company had mad e $0.2 million of benefit payments under the SERP Plan in 2024. The Company presently anticipates making an ad ditional $0.5 million of benefit payments under the SERP Plan in 2024.

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