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BENEFIT PLANS: Defined Benefit Plan, Net Periodic Benefit Cost (Details) - USD ($) $ in Thousands | 12 Months Ended
--- | ---
Dec. 31, 2025 | Dec. 31, 2024 | Dec. 31, 2023
Defined Benefit Plan Disclosure |  |  |  |
Net periodic benefit cost |  | $ 5,986 | $ 8,077 | $ 6,513
Pension Plan |  |  |  |
Defined Benefit Plan Disclosure |  |  |  |
Service cost |  | 31,774 | 33,992 | 29,843
Interest cost |  | 56,151 | 52,181 | 51,277
Expected return on plan assets |  | 68,931 | 66,533 | 61,728
Amortization of net loss |  | 0 | (1,700) | 0
Amortization of prior service cost |  | 6 | 6 | 6
Net periodic benefit cost |  | 19,000 | 21,346 | 19,398
Regulatory deferral of net periodic benfit cost | [1] | 18,159 | 20,425 | 18,553
Previously deferred pension cost recognized | [1] | 35,182 | 35,182 | 17,154
Net periodic benefit cost recognized for financial reporting | [1],[2] | 36,023 | 36,103 | 17,999
Pension and SMSP |  |  |  |
Defined Benefit Plan Disclosure |  |  |  |
NetServiceCostDefinedBenefitPlan |  | 37,800 | 35,900 | 18,200
otherexpensedefinedbenefitplanbenefitcost |  | 6,000 | 8,100 | 6,500
Senior Management Security Plan |  |  |  |
Defined Benefit Plan Disclosure |  |  |  |
Service cost |  | 1,172 | 1,051 | 612
Interest cost |  | 5,640 | 5,332 | 5,322
Expected return on plan assets |  | 0 | 0 | 0
Amortization of net loss |  | (690) | (1,312) | (570)
Amortization of prior service cost |  | 221 | 220 | 219
Net periodic benefit cost |  | 7,723 | 7,915 | 6,723
Regulatory deferral of net periodic benfit cost |  | 0 | 0 | 0
Net periodic benefit cost recognized for financial reporting | [2] | 7,723 | 7,915 | 6,723
Postretirement Benefits |  |  |  |
Defined Benefit Plan Disclosure |  |  |  |
Service cost |  | 672 | 698 | 658
Interest cost |  | 2,973 | 2,824 | 2,980
Expected return on plan assets |  | 1,786 | 1,831 | 1,650
Amortization of net loss |  | 1,765 | 1,494 | 1,237
Amortization of prior service cost |  | 1,374 | 1,548 | 1,665
Net periodic benefit cost |  | $ 1,468 | $ 1,745 | $ 2,416
[1] | 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.
[2] | Of total net periodic pension cost recognized for financial reporting $37.8 million, $35.9 million, and $18.2 million respectively, was recognized in "Other operations and maintenance" and $6.0 million, $8.1 million, and $6.5 million respectively, was recognized in "Other income, net" on the consolidated statements of income of the companies for the twelve months ended December 31, 2025, 2024, and 2023.

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