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Employee Benefit Plans - Schedule of Change in Benefit Obligations (Details) - USD ($)
$ in Millions
 12 Months Ended
Sep. 30, 2025
Sep. 30, 2024
Sep. 30, 2023
Change in benefit obligations      
Actuarial (gain) loss $ 26.6 $ (2.4) $ (41.6)
Amounts in the Consolidated Balance Sheets      
Noncurrent benefit liabilities 187.5 176.2  
Weighted-average plan assumptions      
Noncurrent postemployment benefits liability 0.4 0.4  
Pension benefits      
Change in benefit obligations      
Benefit obligations at beginning of period 1,565.7 1,478.1  
Interest cost 69.9 83.4 81.8
Benefits paid (129.4) (130.6)  
Actuarial (gain) loss (11.9) 140.0  
Transfers in 1.2 1.6  
Settlements (1.2) (6.8)  
Benefit obligations at end of period 1,494.3 1,565.7 1,478.1
Change in plan assets      
Fair value of plan assets at beginning of period 1,452.9 1,361.0  
Actual return on plan assets 33.7 213.5  
Employer contributions 8.3 14.2  
Benefits paid (129.4) (130.6)  
Settlements (1.2) (6.8)  
Transfers in 1.2 1.6  
Fair value of plan assets at end of period 1,365.5 1,452.9 1,361.0
Unfunded status of the plans as of September 30 128.8 112.8  
Amounts in the Consolidated Balance Sheets      
Noncurrent benefit assets 44.9 49.0  
Current benefit liabilities 7.0 7.1  
Noncurrent benefit liabilities 166.7 154.7  
Total benefit liabilities 173.7 161.8  
Net liabilities recognized 128.8 112.8  
Balance in Accumulated other comprehensive loss      
Prior service cost (credit) $ 1.0 $ 1.1  
Weighted-average plan assumptions      
Discount rate 5.22% 4.94%  
Other postretirement benefits      
Change in benefit obligations      
Benefit obligations at beginning of period $ 23.8 $ 22.3  
Interest cost 1.0 1.2 1.2
Benefits paid (2.3) (2.4)  
Actuarial (gain) loss 0.4 2.7  
Transfers in 0.0 0.0  
Settlements 0.0 0.0  
Benefit obligations at end of period 22.9 23.8 22.3
Change in plan assets      
Fair value of plan assets at beginning of period 0.0 0.0  
Actual return on plan assets 0.0 0.0  
Employer contributions 2.3 2.4  
Benefits paid (2.3) (2.4)  
Settlements 0.0 0.0  
Transfers in 0.0 0.0  
Fair value of plan assets at end of period 0.0 0.0 $ 0.0
Unfunded status of the plans as of September 30 22.9 23.8  
Amounts in the Consolidated Balance Sheets      
Noncurrent benefit assets 0.0 0.0  
Current benefit liabilities 2.5 2.7  
Noncurrent benefit liabilities 20.4 21.1  
Total benefit liabilities 22.9 23.8  
Net liabilities recognized 22.9 23.8  
Balance in Accumulated other comprehensive loss      
Prior service cost (credit) $ (12.2) $ (14.4)  
Weighted-average plan assumptions      
Discount rate 5.14% 4.89%  
Assumed pre-65 health care cost trend rate 6.90% 7.20%  
Ultimate pre-65 health care cost trend rate 4.00%