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Employee Benefit Plans - Schedule of Amortization of Prior Service Cost (Credit) Recognized in AOCI (Details) - USD ($)
$ in Millions
 12 Months Ended
Sep. 30, 2025
Sep. 30, 2024
Sep. 30, 2023
Pension benefits      
Defined Benefit Plan Disclosure [Line Items]      
Amortization of prior service credits (costs) recognized in Accumulated other comprehensive income $ (0.1) $ (0.1) $ (0.1)
Net periodic benefit costs (income) 24.4 10.0 (20.0)
Total pre-tax amount recognized in Comprehensive income 24.3 9.9 (20.1)
Other postretirement benefits      
Defined Benefit Plan Disclosure [Line Items]      
Amortization of prior service credits (costs) recognized in Accumulated other comprehensive income 2.2 2.2 2.2
Net periodic benefit costs (income) (0.8) 1.7 (7.6)
Total pre-tax amount recognized in Comprehensive income $ 1.4 $ 3.9 $ (5.4)