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Employee Benefit Plans: Components of Net Periodic Expense (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2023
Dec. 31, 2022
Dec. 31, 2021
Pension Plan      
Defined Benefit Plan Disclosure [Line Items]      
Service Cost $ 2.5 $ 3.9 $ 5.0
Interest cost 17.5 10.8 9.3
Expected return on assets (18.7) (18.5) (20.8)
Net amortization of prior service cost (0.1) (0.1) 0.0
Recognized net actuarial loss (gain) 2.0 6.1 7.3
Net periodic benefit expense 3.2 2.2 0.8
Supplemental Employee Retirement Plan      
Defined Benefit Plan Disclosure [Line Items]      
Service Cost 3.1 (0.8) 3.1
Interest cost 1.5 0.8 0.7
Expected return on assets 0.0 0.0 0.0
Net amortization of prior service cost 0.0 0.0 0.0
Recognized net actuarial loss (gain) 0.0 0.3 1.8
Net periodic benefit expense 4.6 0.3 5.6
Other Postretirement Benefits Plan      
Defined Benefit Plan Disclosure [Line Items]      
Service Cost 1.5 1.9 2.2
Interest cost 2.4 1.3 1.0
Expected return on assets (0.2) (0.1) (0.1)
Net amortization of prior service cost 0.0 (0.3) (0.4)
Recognized net actuarial loss (gain) 0.0 0.1 0.5
Net periodic benefit expense $ 3.7 $ 2.9 $ 3.2