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Employee Benefit Plans: Components of Net Periodic Expense (Details) - USD ($)
$ in Millions
 3 Months Ended
Mar. 31, 2024
Mar. 31, 2023
Pension Plan    
Defined Benefit Plan Disclosure [Line Items]    
Service Cost $ 0.6 $ 0.6
Interest cost 4.1 4.4
Expected return on plan assets (4.5) (4.7)
Net amortization of prior service costs 0.0 0.0
Recognized net actuarial loss (gain) 0.5 0.5
Net periodic expense (benefit) 0.7 0.8
Supplemental Employee Retirement Plan    
Defined Benefit Plan Disclosure [Line Items]    
Service Cost 1.0 0.9
Interest cost 0.3 0.4
Expected return on plan assets 0.0 0.0
Net amortization of prior service costs 0.0 0.0
Recognized net actuarial loss (gain) 0.0 0.0
Net periodic expense (benefit) 1.3 1.3
Other Postretirement Benefits Plan    
Defined Benefit Plan Disclosure [Line Items]    
Service Cost 0.4 0.4
Interest cost 0.6 0.6
Expected return on plan assets (0.1) (0.1)
Net amortization of prior service costs 0.1 0.0
Recognized net actuarial loss (gain) 0.0 0.0
Net periodic expense (benefit) $ 1.0 $ 0.9