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Employee Benefit Plans: Fair Value Measurements (Details) - USD ($) $ in Millions | 12 Months Ended |  |
--- | --- | --- | ---
Dec. 31, 2023 | Dec. 31, 2022 | Dec. 31, 2021
Pension Plan |  |  |
Defined Benefit Plan, Plan Assets, Amount | $ 308.6 | $ 323.1 | $ 458.4
Fair Value, Measurements, Recurring | Pension Plan |  |  |
Defined Benefit Plan, Plan Assets, Amount | 308.4 | 323.1 |
Defined Benefit Plan, Fair Value of Plan Assets, Excluding Net Asset Value investments | 283.9 | 293.5 |
Defined Benefit Plan, Alternative Investments, Fair Value Of Plan Assets | 24.5 | 29.6 |
Fair Value, Measurements, Recurring | Postretirement Health Coverage |  |  |
Defined Benefit Plan, Plan Assets, Amount | 8.0 | 7.8 |
Defined Benefit Plan, Fair Value of Plan Assets, Excluding Net Asset Value investments | 8.0 | 7.8 |
Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring | Pension Plan |  |  |
Defined Benefit Plan, Plan Assets, Amount |  |  |
Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring | Postretirement Health Coverage |  |  |
Defined Benefit Plan, Plan Assets, Amount | 8.0 | 7.8 |
Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring | Pension Plan |  |  |
Defined Benefit Plan, Plan Assets, Amount | 283.9 | 293.5 |
Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring | Postretirement Health Coverage |  |  |
Defined Benefit Plan, Plan Assets, Amount |  |  |
Fair Value, Inputs, Level 3 | Fair Value, Measurements, Recurring | Pension Plan |  |  |
Defined Benefit Plan, Plan Assets, Amount |  |  |
Fair Value, Inputs, Level 3 | Fair Value, Measurements, Recurring | Postretirement Health Coverage |  |  |
Defined Benefit Plan, Plan Assets, Amount |  |  |
Common Collective Trust, Cash And Cash Equivalents | Fair Value, Measurements, Recurring | Pension Plan |  |  |
Defined Benefit Plan, Plan Assets, Amount | 6.7 | 6.4 |
Defined Benefit Plan, Fair Value of Plan Assets, Excluding Net Asset Value investments | 6.7 | 6.4 |
Defined Benefit Plan, Alternative Investments, Fair Value Of Plan Assets |  |  |
Common Collective Trust, Cash And Cash Equivalents | Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring | Pension Plan |  |  |
Defined Benefit Plan, Plan Assets, Amount |  |  |
Common Collective Trust, Cash And Cash Equivalents | Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring | Pension Plan |  |  |
Defined Benefit Plan, Plan Assets, Amount | 6.7 | 6.4 |
Common Collective Trust, Cash And Cash Equivalents | Fair Value, Inputs, Level 3 | Fair Value, Measurements, Recurring | Pension Plan |  |  |
Defined Benefit Plan, Plan Assets, Amount |  |  |
Common Collective Trust - Equity | Fair Value, Measurements, Recurring | Pension Plan |  |  |
Defined Benefit Plan, Plan Assets, Amount | 42.7 | 45.1 |
Defined Benefit Plan, Fair Value of Plan Assets, Excluding Net Asset Value investments | 42.7 | 45.1 |
Defined Benefit Plan, Alternative Investments, Fair Value Of Plan Assets |  |  |
Common Collective Trust - Equity | Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring | Pension Plan |  |  |
Defined Benefit Plan, Plan Assets, Amount |  |  |
Common Collective Trust - Equity | Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring | Pension Plan |  |  |
Defined Benefit Plan, Plan Assets, Amount | 42.7 | 45.1 |
Common Collective Trust - Equity | Fair Value, Inputs, Level 3 | Fair Value, Measurements, Recurring | Pension Plan |  |  |
Defined Benefit Plan, Plan Assets, Amount |  |  |
Common Collective Trust - Fixed Income | Fair Value, Measurements, Recurring | Pension Plan |  |  |
Defined Benefit Plan, Plan Assets, Amount | 234.5 | 242.0 |
Defined Benefit Plan, Fair Value of Plan Assets, Excluding Net Asset Value investments | 234.5 | 242.0 |
Defined Benefit Plan, Alternative Investments, Fair Value Of Plan Assets |  |  |
Common Collective Trust - Fixed Income | Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring | Pension Plan |  |  |
Defined Benefit Plan, Plan Assets, Amount |  |  |
Common Collective Trust - Fixed Income | Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring | Pension Plan |  |  |
Defined Benefit Plan, Plan Assets, Amount | 234.5 | 242.0 |
Common Collective Trust - Fixed Income | Fair Value, Inputs, Level 3 | Fair Value, Measurements, Recurring | Pension Plan |  |  |
Defined Benefit Plan, Plan Assets, Amount |  |  |
Common Collective Trust - Real Estate | Fair Value, Measurements, Recurring | Pension Plan |  |  |
Defined Benefit Plan, Plan Assets, Amount | 16.4 | 21.5 |
Defined Benefit Plan, Fair Value of Plan Assets, Excluding Net Asset Value investments |  |  |
Defined Benefit Plan, Alternative Investments, Fair Value Of Plan Assets | 16.4 | 21.5 |
Common Collective Trust - Real Estate | Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring | Pension Plan |  |  |
Defined Benefit Plan, Plan Assets, Amount |  |  |
Common Collective Trust - Real Estate | Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring | Pension Plan |  |  |
Defined Benefit Plan, Plan Assets, Amount |  |  |
Common Collective Trust - Real Estate | Fair Value, Inputs, Level 3 | Fair Value, Measurements, Recurring | Pension Plan |  |  |
Defined Benefit Plan, Plan Assets, Amount |  |  |
Hedge Funds | Minimum |  |  |
Percentage Of Monthly Redemption | 10.00% |  |
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share, Investment Redemption, Notice Period | 15 days |  |
Hedge Funds | Maximum |  |  |
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share, Investment Redemption, Notice Period | 60 days |  |
Hedge Funds | Fair Value, Measurements, Recurring | Pension Plan |  |  |
Defined Benefit Plan, Plan Assets, Amount | $ 8.1 | 8.1 |
Defined Benefit Plan, Fair Value of Plan Assets, Excluding Net Asset Value investments |  |  |
Defined Benefit Plan, Alternative Investments, Fair Value Of Plan Assets | 8.1 | 8.1 |
Hedge Funds | Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring | Pension Plan |  |  |
Defined Benefit Plan, Plan Assets, Amount |  |  |
Hedge Funds | Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring | Pension Plan |  |  |
Defined Benefit Plan, Plan Assets, Amount |  |  |
Hedge Funds | Fair Value, Inputs, Level 3 | Fair Value, Measurements, Recurring | Pension Plan |  |  |
Defined Benefit Plan, Plan Assets, Amount |  |  |
Cash | Fair Value, Measurements, Recurring | Postretirement Health Coverage |  |  |
Defined Benefit Plan, Plan Assets, Amount | 8.0 |  |
Defined Benefit Plan, Fair Value of Plan Assets, Excluding Net Asset Value investments | 8.0 |  |
Cash | Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring | Postretirement Health Coverage |  |  |
Defined Benefit Plan, Plan Assets, Amount | 8.0 |  |
Cash | Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring | Postretirement Health Coverage |  |  |
Defined Benefit Plan, Plan Assets, Amount |  |  |
Cash | Fair Value, Inputs, Level 3 | Fair Value, Measurements, Recurring | Postretirement Health Coverage |  |  |
Defined Benefit Plan, Plan Assets, Amount |  |  |

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