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Employee Benefit Plans - Pension and Other Postretirement Benefit Costs (Details) - USD ($)
$ in Millions
 12 Months Ended
Sep. 30, 2025
Sep. 30, 2024
Sep. 30, 2023
Pension benefits      
Net periodic benefit costs (income)      
Interest cost $ 69.9 $ 83.4 $ 81.8
Expected return on plan assets (71.8) (68.4) (66.9)
Amortization of prior service costs (credits) 0.1 0.1 0.1
Actuarial loss (gain) 26.2 (5.1) (35.0)
Net periodic benefit costs (income) $ 24.4 $ 10.0 $ (20.0)
Weighted-average plan assumptions      
Discount rate for interest cost 4.65% 5.92% 5.45%
Expected long-term rate of return on plan assets 5.20% 5.30% 4.90%
Other postretirement benefits      
Net periodic benefit costs (income)      
Interest cost $ 1.0 $ 1.2 $ 1.2
Expected return on plan assets 0.0 0.0 0.0
Amortization of prior service costs (credits) (2.2) (2.2) (2.2)
Actuarial loss (gain) 0.4 2.7 (6.6)
Net periodic benefit costs (income) $ (0.8) $ 1.7 $ (7.6)
Weighted-average plan assumptions      
Discount rate for interest cost 4.63% 5.92% 5.41%