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Employee Benefit Plans - Components of Pension and Other Postretirement Benefit Income (Details) - USD ($)
$ in Millions
 3 Months Ended 6 Months Ended
Mar. 31, 2026
Mar. 31, 2025
Mar. 31, 2026
Mar. 31, 2025
Pension benefits        
Defined Benefit Plan Disclosure [Line Items]        
Interest cost $ 17.0 $ 17.5 $ 33.9 $ 34.9
Expected return on plan assets (17.9) (18.1) (35.7) (36.1)
Amortization of prior service credits 0.0 0.0 0.0 0.0
Net periodic benefit income (0.9) (0.6) (1.8) (1.2)
Other postretirement benefits        
Defined Benefit Plan Disclosure [Line Items]        
Interest cost 0.2 0.2 0.5 0.5
Expected return on plan assets 0.0 0.0 0.0 0.0
Amortization of prior service credits (0.5) (0.5) (1.1) (1.1)
Net periodic benefit income $ (0.3) $ (0.3) $ (0.6) $ (0.6)