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BENEFIT PLANS: Defined Benefit Plan, Net Periodic Benefit Cost (Details) - USD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2023 | Dec. 31, 2022 | Dec. 31, 2021
Defined Benefit Plan Disclosure |  |  |  |
Net periodic benefit cost | [1] | $ 6,513 | $ 9,196 | $ 15,249
Pension Plan |  |  |  |
Defined Benefit Plan Disclosure |  |  |  |
Service cost |  | 29,843 | 52,025 | 54,202
Interest cost |  | 51,277 | 39,670 | 37,317
Expected return on plan assets |  | 61,728 | 72,348 | 64,090
Amortization of net loss |  | 0 | (12,273) | (23,796)
Amortization of prior service cost |  | 6 | 6 | 6
Net periodic benefit cost |  | 19,398 | 31,626 | 51,231
Regulatory deferral of net periodic benfit cost | [2] | 18,553 | 30,197 | 48,962
Previously deferred pension cost recognized | [2] | 17,154 | 17,154 | 17,154
Net periodic benefit cost recognized for financial reporting | [2],[3] | 17,999 | 18,583 | 19,423
Pension and SMSP |  |  |  |
Defined Benefit Plan Disclosure |  |  |  |
NetServiceCostDefinedBenefitPlan |  | 18,200 | 19,000 | 17,800
otherexpensedefinedbenefitplanbenefitcost |  | 6,500 | 9,200 | 10,500
Senior Management Security Plan |  |  |  |
Defined Benefit Plan Disclosure |  |  |  |
Service cost |  | 612 | 1,185 | 813
Interest cost |  | 5,322 | 3,897 | 3,557
Expected return on plan assets |  | 0 | 0 | 0
Amortization of net loss |  | (570) | (4,229) | (4,205)
Amortization of prior service cost |  | 219 | 279 | 296
Net periodic benefit cost |  | 6,723 | 9,590 | 8,871
Regulatory deferral of net periodic benfit cost |  | 0 | 0 | 0
Net periodic benefit cost recognized for financial reporting | [3] | 6,723 | 9,590 | 8,871
Postretirement Benefits |  |  |  |
Defined Benefit Plan Disclosure |  |  |  |
Service cost |  | 658 | 1,071 | 1,063
Interest cost |  | 2,980 | 2,112 | 2,059
Expected return on plan assets |  | 1,650 | 2,351 | 2,395
Amortization of net loss |  | 1,237 | 31 | 0
Amortization of prior service cost |  | 1,665 | 295 | 47
Net periodic benefit cost |  | 2,416 | 1,096 | $ 5,510
Defined Benefit Plan, Accumulated Benefit Obligation, Increase (Decrease) for Plan Amendment |  | $ 0 | $ 8,065 |
[1] | The 2021 pension and postretirement non-service costs includes $4.7 million of expense for a temporary deviation from the cost-sharing provisions of the substantive postretirement plan as described in Note 11 - "Benefit Plans."
[2] | Net periodic pension costs for the pension plan are recognized for financial reporting based upon the authorization of each regulatory jurisdiction in which Idaho Power operates. Under an IPUC order, the Idaho portion of net periodic pension cost is recorded as a regulatory asset and is recognized in the income statement as those costs are recovered through rates.
[3] | Of total net periodic pension cost recognized for financial reporting $18.2 million, $19.0 million, and $17.8 million respectively, was recognized in "Other operations and maintenance" and $6.5 million, $9.2 million, and $10.5 million respectively, was recognized in "Other (income) expense, net" on the consolidated statements of income of the companies for the twelve months ended December 31, 2023, 2022, and 2021.

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