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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