v3.25.4
Employee Benefit Plans: Employee Benefit Plan Obligations (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2025
Dec. 31, 2024
Dec. 31, 2023
Amounts recognized on our Consolidated Balance Sheets as of December 31:      
Regulatory assets $ 394.7 $ 427.7  
Non-current liabilities 123.4 122.9  
Regulatory liabilities 588.2 568.7  
Pension Plan      
Defined Benefit Plan Disclosure [Line Items]      
Accumulated benefit obligation at end of year 311.8 307.1  
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]      
Benefit obligation at beginning of year 311.6 348.1  
Service Cost 1.7 2.3 $ 2.5
Interest cost 16.0 16.4 17.5
Actuarial (gain) loss 10.6 (16.0)  
Benefits paid (23.4) (39.2)  
Plan participants’ contributions 0.0 0.0  
Benefit obligation at end of year 316.5 311.6 348.1
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]      
Fair value of plan assets at beginning of year 270.2 308.6  
Investment income (loss) 25.7 (1.6)  
Employer contributions 1.8 2.3  
Retiree contributions 0.0 0.0  
Benefits paid (23.4) (39.1)  
Fair value of plan assets at end of year 274.3 270.2 308.6
Funded status - deficiency 42.2 41.4  
Amounts recognized on our Consolidated Balance Sheets as of December 31:      
Regulatory assets 81.7 81.6  
Current liabilities 0.0 0.0  
Non-current liabilities 42.2 41.4  
Regulatory liabilities 3.4 3.1  
Amounts recognized in AOCI, net of tax as of December 31:      
Net (gain) loss 5.1 5.2  
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 5.1 5.2  
Supplemental Plans      
Defined Benefit Plan Disclosure [Line Items]      
Accumulated benefit obligation at end of year 47.4 46.1  
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]      
Benefit obligation at beginning of year 46.1 46.7  
Service Cost 3.6 3.3 3.1
Interest cost 1.4 1.4 1.5
Actuarial (gain) loss 0.8 (1.2)  
Benefits paid (4.5) (4.1)  
Plan participants’ contributions 0.0 0.0  
Benefit obligation at end of year 47.4 46.1 46.7
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 4.5 4.1  
Retiree contributions 0.0 0.0  
Benefits paid (4.5) (4.1)  
Fair value of plan assets at end of year 0.0 0.0 0.0
Funded status - deficiency 47.4 46.1  
Amounts recognized on our Consolidated Balance Sheets as of December 31:      
Regulatory assets 0.0 0.0  
Current liabilities 2.8 2.8  
Non-current liabilities 44.6 43.3  
Regulatory liabilities 0.0 0.0  
Amounts recognized in AOCI, net of tax as of December 31:      
Net (gain) loss 1.5 0.8  
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 1.5 0.8  
Healthcare Plan      
Defined Benefit Plan Disclosure [Line Items]      
Accumulated benefit obligation at end of year 47.9 48.5  
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]      
Benefit obligation at beginning of year 48.5 51.1  
Service Cost 1.5 1.6 1.5
Interest cost 2.4 2.4 2.4
Actuarial (gain) loss (0.7) (1.7)  
Benefits paid (4.6) (5.7)  
Plan participants’ contributions 0.8 0.8  
Benefit obligation at end of year 47.9 48.5 51.1
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]      
Fair value of plan assets at beginning of year 7.5 8.0  
Investment income (loss) 0.1 0.1  
Employer contributions 3.3 4.2  
Retiree contributions 0.8 0.9  
Benefits paid (4.6) (5.7)  
Fair value of plan assets at end of year 7.1 7.5 $ 8.0
Funded status - deficiency 40.8 41.0  
Amounts recognized on our Consolidated Balance Sheets as of December 31:      
Regulatory assets 5.1 4.9  
Current liabilities 4.1 4.3  
Non-current liabilities 36.6 36.7  
Regulatory liabilities 8.3 7.3  
Amounts recognized in AOCI, net of tax as of December 31:      
Net (gain) loss (0.7) (0.7)  
Prior service cost (gain) 0.0 0.0  
Total amounts included in AOCI, net of tax not yet recognized as components of net periodic expense $ (0.7) $ (0.7)