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Employee Benefit Plans: Components of Net Periodic Expense (Details) - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended
Sep. 30, 2025
Sep. 30, 2024
Sep. 30, 2025
Sep. 30, 2024
Defined Benefit Pension Plans        
Defined Benefit Plan Disclosure [Line Items]        
Service Cost $ 0.4 $ 0.6 $ 1.3 $ 1.7
Interest cost 4.0 4.1 12.0 12.3
Expected return on plan assets (4.2) (4.5) (12.7) (13.4)
Net amortization of prior service costs 0.0 0.0 (0.1) (0.1)
Recognized net actuarial loss 0.5 0.5 1.6 1.5
Net periodic expense 0.7 0.7 2.1 2.0
Supplemental Non-Qualified Defined Benefit Plans        
Defined Benefit Plan Disclosure [Line Items]        
Service Cost 1.2 1.3 2.8 2.9
Interest cost 0.3 0.4 1.1 1.1
Expected return on plan assets 0.0 0.0 0.0 0.0
Net amortization of prior service costs 0.0 0.0 0.0 0.0
Recognized net actuarial loss 0.0 0.0 0.0 0.0
Net periodic expense 1.5 1.7 3.9 4.0
Non-pension Defined Benefit Postretirement Healthcare Plan        
Defined Benefit Plan Disclosure [Line Items]        
Service Cost 0.4 0.4 1.1 1.2
Interest cost 0.6 0.6 1.8 1.8
Expected return on plan assets (0.1) (0.1) (0.2) (0.2)
Net amortization of prior service costs 0.1 0.1 0.2 0.1
Recognized net actuarial loss 0.0 0.0 0.0 0.0
Net periodic expense $ 1.0 $ 1.0 $ 2.9 $ 2.9