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Employee Benefit Plans: Components of Net Periodic Expense (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2023 | Dec. 31, 2022 | Dec. 31, 2021
Pension Plan |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Service Cost | $ 2.5 | $ 3.9 | $ 5.0
Interest cost | 17.5 | 10.8 | 9.3
Expected return on assets | (18.7) | (18.5) | (20.8)
Net amortization of prior service cost | (0.1) | (0.1) | 0.0
Recognized net actuarial loss (gain) | 2.0 | 6.1 | 7.3
Net periodic benefit expense | 3.2 | 2.2 | 0.8
Supplemental Employee Retirement Plan |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Service Cost | 3.1 | (0.8) | 3.1
Interest cost | 1.5 | 0.8 | 0.7
Expected return on assets | 0.0 | 0.0 | 0.0
Net amortization of prior service cost | 0.0 | 0.0 | 0.0
Recognized net actuarial loss (gain) | 0.0 | 0.3 | 1.8
Net periodic benefit expense | 4.6 | 0.3 | 5.6
Other Postretirement Benefits Plan |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Service Cost | 1.5 | 1.9 | 2.2
Interest cost | 2.4 | 1.3 | 1.0
Expected return on assets | (0.2) | (0.1) | (0.1)
Net amortization of prior service cost | 0.0 | (0.3) | (0.4)
Recognized net actuarial loss (gain) | 0.0 | 0.1 | 0.5
Net periodic benefit expense | $ 3.7 | $ 2.9 | $ 3.2

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- Definition Amount of prior service cost (credit) recognized in net periodic benefit cost (credit) of defined benefit plan. + 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 45 -Paragraph 3A -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480535/715-20-45-3A Reference 3: http://www.xbrl.org/2003/role/exampleRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 55 -Paragraph 18 -Publisher FASB -URI https://asc.fasb.org//1943274/2147480482/715-20-55-18 Reference 4: http://www.xbrl.org/2003/role/disclosureRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (h)(5) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480506/715-20-50-1 Reference 5: http://www.xbrl.org/2003/role/disclosureRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 6 -Subparagraph (a)(5) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480506/715-20-50-6 + Details
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- Definition Amount of expected return (loss) recognized in net periodic benefit (cost) credit, calculated based on expected long-term rate of return and market-related value of plan assets of defined benefit plan. + 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 45 -Paragraph 3A -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480535/715-20-45-3A Reference 3: http://www.xbrl.org/2003/role/exampleRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 55 -Paragraph 18 -Publisher FASB -URI https://asc.fasb.org//1943274/2147480482/715-20-55-18 Reference 4: http://www.xbrl.org/2003/role/disclosureRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (h)(3) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480506/715-20-50-1 Reference 5: http://www.xbrl.org/2003/role/disclosureRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 6 -Subparagraph (a)(3) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480506/715-20-50-6 + Details
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