v3.25.4
Employee Benefit Plans: Components of Net Periodic Expense (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2025
Dec. 31, 2024
Dec. 31, 2023
Pension Plan      
Defined Benefit Plan Disclosure [Line Items]      
Service Cost $ 1.7 $ 2.3 $ 2.5
Interest cost 16.0 16.4 17.5
Expected return on assets (17.0) (18.0) (18.7)
Net amortization of prior service cost (0.1) (0.1) (0.1)
Recognized net actuarial loss (gain) 2.2 2.0 2.0
Net periodic benefit expense 2.8 2.6 3.2
Supplemental Plans      
Defined Benefit Plan Disclosure [Line Items]      
Service Cost 3.6 3.3 3.1
Interest cost 1.4 1.4 1.5
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 0.0 0.0
Net periodic benefit expense 5.0 4.7 4.6
Healthcare Plan      
Defined Benefit Plan Disclosure [Line Items]      
Service Cost 1.5 1.6 1.5
Interest cost 2.4 2.4 2.4
Expected return on assets (0.3) (0.3) (0.2)
Net amortization of prior service cost 0.2 0.2 0.0
Recognized net actuarial loss (gain) 0.0 0.0 0.0
Net periodic benefit expense $ 3.8 $ 3.9 $ 3.7