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BENEFIT PLANS: Schedule of Defined Benefit Plan Disclosures, Other Postretirment Benefit Plans (Details) - USD ($) $ in Thousands | 12 Months Ended
--- | ---
Dec. 31, 2023 | Dec. 31, 2022 | Dec. 31, 2021
Change in Fair Value of Plan Assets [Roll Forward] |  |  |  |  |  |
Employer contributions |  | $ (9,800) |  | $ (8,800) |  | $ (8,200)
Supplemental Unemployment Benefits, Disability Related Benefits |  | 3,000 |  | 3,000 |  |
Postretirement Benefits |  |  |  |  |  |
Change in Benefit Obligation [Roll Forward] |  |  |  |  |  |
Benefit obligation at January 1 |  | 59,099 |  | 74,075 |  |
Service cost |  | 658 |  | 1,071 |  | 1,063
Interest cost |  | 2,980 |  | 2,112 |  | 2,059
Actuarial loss (gain) |  | (2,004) |  | (21,845) |  |
Defined Benefit Plan, Benefit Obligation, Benefits Paid | [1] | (4,669) |  | (4,379) |  |
Defined Benefit Plan, Accumulated Benefit Obligation, Increase (Decrease) for Plan Amendment |  | 0 |  | 8,065 |  |
Projected benefit obligation at December 31 |  | 56,064 |  | 59,099 |  | 74,075
Change in Fair Value of Plan Assets [Roll Forward] |  |  |  |  |  |
Fair value at January 1 |  | 28,565 | [2] | 41,464 |  |
Actual return (loss) on plan assets |  | 7,219 |  | (6,586) |  |
Employer contributions | [1] | (690) |  | (1,934) |  |
Benefits Paid | [1] | (4,670) |  | (4,379) |  |
Fair value at December 31 |  | 31,804 | [2] | 28,565 | [2] | $ 41,464
Funded status at end of year |  | (24,260) |  | (30,534) |  |
Defined Benefit Plan, Plan Assets, Contributions by Plan Participant |  | $ 2,600 |  | $ 2,900 |  |
Postretirement Benefit, Medical trend rate EY |  | 7.10% |  | 6.70% |  |
[1] | Contributions and benefits paid are each net of $2.6 million and $2.9 million of plan participant contributions for 2023 and 2022, respectively.
[2] | The postretirement benefits assets are primarily life insurance contracts.

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- Definition Amount of increase (decrease) in accumulated benefit obligation from change in existing term of defined benefit plan or initiation of new defined benefit plan. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (r) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480506/715-20-50-1 + Details
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- Definition Amount of increase (decrease) in plan assets of defined benefit plan from actual return (loss) determined by change in fair value of plan assets adjusted for contributions, benefit payments, and other expenses. + References Reference 1: http://www.xbrl.org/2003/role/exampleRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 55 -Paragraph 17 -Publisher FASB -URI https://asc.fasb.org//1943274/2147480482/715-20-55-17 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (b)(1) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480506/715-20-50-1 + Details
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- Definition Amount of payment to participant of defined benefit plan which decreases benefit obligation. For pension plan, payment includes, but is not limited to, pension benefits and death benefits. For other postretirement plan, payment includes, but is not limited to, prescription drug benefits, health care benefits, life insurance benefits, and legal, educational and advisory services. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 715 -SubTopic 60 -Name Accounting Standards Codification -Section 50 -Paragraph 4 -Publisher FASB -URI https://asc.fasb.org//1943274/2147480266/715-60-50-4 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (a)(6) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480506/715-20-50-1 + Details
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