Employee Benefit Plans: Employee Benefit Plan Obligations (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Amounts recognized in the balance sheet as of December 31 consist of:
Regulatory assets	$ 480.1	$ 653.0
Non-current liabilities	123.9	116.7
Regulatory liabilities	566.6	518.6
Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligation at end of year	341.8	350.2
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Projected benefit obligation at beginning of year	358.4	478.3
Service Cost	2.5	3.9	$ 5.0
Interest cost	17.5	10.8	9.3
Actuarial (gain) loss	11.6	(97.9)
Benefits paid	(41.9)	(36.7)
Plan participants’ contributions	0.0	0.0
Projected benefit obligation at end of year	348.1	358.4	478.3
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	323.1	458.4
Investment income (loss)	27.4	(98.6)
Employer contributions	0.0	0.0
Retiree contributions	0.0	0.0
Benefits paid	(41.9)	(36.7)
Fair value of plan assets at end of year	308.6	323.1	458.4
Funded status - deficiency	39.5	35.3
Amounts recognized in the balance sheet as of December 31 consist of:
Regulatory assets	79.9	78.7
Current liabilities	0.0	0.0
Non-current assets	0.0	0.0
Non-current liabilities	39.4	35.2
Regulatory liabilities	2.9	2.8
Amounts recognized in accumulated OCI (after-tax) as of December 31 consist of:
Net (gain) loss	5.0	5.2
Prior service cost (gain)	0.0	(0.1)
Total amounts included in AOCI, after-tax not yet recognized as components of net periodic expense	5.0	5.1
Supplemental Employee Retirement Plan
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligation at end of year	46.7	45.2
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Projected benefit obligation at beginning of year	45.2	55.3
Service Cost	3.1	(0.8)	3.1
Interest cost	1.5	0.8	0.7
Actuarial (gain) loss	0.3	(7.0)
Benefits paid	(3.4)	(3.1)
Plan participants’ contributions	0.0	0.0
Projected benefit obligation at end of year	46.7	45.2	55.3
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	3.5	3.1
Retiree contributions	0.0	0.0
Benefits paid	(3.5)	(3.1)
Fair value of plan assets at end of year	0.0	0.0	0.0
Funded status - deficiency	46.7	45.2
Amounts recognized in the balance sheet as of December 31 consist of:
Regulatory assets	0.0	0.0
Current liabilities	2.4	2.2
Non-current assets	0.0	0.0
Non-current liabilities	44.3	43.0
Regulatory liabilities	0.0	0.0
Amounts recognized in accumulated OCI (after-tax) as of December 31 consist of:
Net (gain) loss	1.8	1.6
Prior service cost (gain)	0.0	0.0
Total amounts included in AOCI, after-tax not yet recognized as components of net periodic expense	1.8	1.6
Other Postretirement Benefits Plan
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligation at end of year	51.1	49.7
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Projected benefit obligation at beginning of year	49.7	63.5
Service Cost	1.5	1.9	2.2
Interest cost	2.4	1.3	1.0
Actuarial (gain) loss	1.7	(12.3)
Benefits paid	(5.3)	(6.1)
Plan participants’ contributions	1.1	1.4
Projected benefit obligation at end of year	51.1	49.7	63.5
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	7.8	8.0
Investment income (loss)	0.2
Employer contributions	4.3	4.5
Retiree contributions	1.1	1.4
Benefits paid	(5.4)	(6.1)
Fair value of plan assets at end of year	8.0	7.8	$ 8.0
Funded status - deficiency	43.1	41.9
Amounts recognized in the balance sheet as of December 31 consist of:
Regulatory assets	4.8	3.8
Current liabilities	4.2	4.4
Non-current assets	1.3	1.0
Non-current liabilities	40.2	38.5
Regulatory liabilities	5.5	6.2
Amounts recognized in accumulated OCI (after-tax) as of December 31 consist of:
Net (gain) loss	(0.7)	(0.7)
Prior service cost (gain)	0.1	0.1
Total amounts included in AOCI, after-tax not yet recognized as components of net periodic expense	$ (0.6)	$ (0.6)