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Employee Benefit Plans: Employee Benefit Plan Obligations (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2024
Dec. 31, 2023
Dec. 31, 2022
Amounts recognized on our Consolidated Balance Sheets as of December 31:      
Regulatory assets $ 427.7 $ 480.1  
Non-current liabilities 122.9 123.9  
Regulatory liabilities 568.7 566.6  
Pension Plan      
Defined Benefit Plan Disclosure [Line Items]      
Accumulated benefit obligation at end of year 307.1 341.8  
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]      
Projected benefit obligation at beginning of year 348.1 358.4  
Service Cost 2.3 2.5 $ 3.9
Interest cost 16.4 17.5 10.8
Actuarial (gain) loss (16.0) 11.6  
Benefits paid (39.2) (41.9)  
Plan participants’ contributions 0.0 0.0  
Projected benefit obligation at end of year 311.6 348.1 358.4
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]      
Fair value of plan assets at beginning of year 308.6 323.1  
Investment income (loss) (1.6) 27.4  
Employer contributions 2.3 0.0  
Retiree contributions 0.0 0.0  
Benefits paid (39.1) (41.9)  
Fair value of plan assets at end of year 270.2 308.6 323.1
Funded status - deficiency 41.4 39.5  
Amounts recognized on our Consolidated Balance Sheets as of December 31:      
Regulatory assets 81.6 79.9  
Current liabilities 0.0 0.0  
Non-current assets 0.0 0.0  
Non-current liabilities 41.4 39.4  
Regulatory liabilities 3.1 2.9  
Amounts recognized in AOCI, net of tax as of December 31:      
Net (gain) loss 5.2 5.0  
Prior service cost (gain) 0.0 0.0  
Total amounts included in AOCI, net of tax not yet recognized as components of net periodic expense 5.2 5.0  
Supplemental Plans      
Defined Benefit Plan Disclosure [Line Items]      
Accumulated benefit obligation at end of year 46.1 46.7  
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]      
Projected benefit obligation at beginning of year 46.7 45.2  
Service Cost 3.3 3.1 (0.8)
Interest cost 1.4 1.5 0.8
Actuarial (gain) loss (1.2) 0.3  
Benefits paid (4.1) (3.4)  
Plan participants’ contributions 0.0 0.0  
Projected benefit obligation at end of year 46.1 46.7 45.2
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]      
Fair value of plan assets at beginning of year 0.0 0.0  
Investment income (loss) 0.0 0.0  
Employer contributions 4.1 3.5  
Retiree contributions 0.0 0.0  
Benefits paid (4.1) (3.5)  
Fair value of plan assets at end of year 0.0 0.0 0.0
Funded status - deficiency 46.1 46.7  
Amounts recognized on our Consolidated Balance Sheets as of December 31:      
Regulatory assets 0.0 0.0  
Current liabilities 2.8 2.4  
Non-current assets 0.0 0.0  
Non-current liabilities 43.3 44.3  
Regulatory liabilities 0.0 0.0  
Amounts recognized in AOCI, net of tax as of December 31:      
Net (gain) loss 0.8 1.8  
Prior service cost (gain) 0.0 0.0  
Total amounts included in AOCI, net of tax not yet recognized as components of net periodic expense 0.8 1.8  
Healthcare Plan      
Defined Benefit Plan Disclosure [Line Items]      
Accumulated benefit obligation at end of year 48.5 51.1  
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]      
Projected benefit obligation at beginning of year 51.1 49.7  
Service Cost 1.6 1.5 1.9
Interest cost 2.4 2.4 1.3
Actuarial (gain) loss (1.7) 1.7  
Benefits paid (5.7) (5.3)  
Plan participants’ contributions 0.8 1.1  
Projected benefit obligation at end of year 48.5 51.1 49.7
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]      
Fair value of plan assets at beginning of year 8.0 7.8  
Investment income (loss) 0.1 0.2  
Employer contributions 4.2 4.3  
Retiree contributions 0.9 1.1  
Benefits paid (5.7) (5.4)  
Fair value of plan assets at end of year 7.5 8.0 $ 7.8
Funded status - deficiency 41.0 43.1  
Amounts recognized on our Consolidated Balance Sheets as of December 31:      
Regulatory assets 4.9 4.8  
Current liabilities 4.3 4.2  
Non-current assets 0.0 1.3  
Non-current liabilities 36.7 40.2  
Regulatory liabilities 7.3 5.5  
Amounts recognized in AOCI, net of tax as of December 31:      
Net (gain) loss (0.7) (0.7)  
Prior service cost (gain) 0.0 0.1  
Total amounts included in AOCI, net of tax not yet recognized as components of net periodic expense $ (0.7) $ (0.6)