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Stockholders' Equity - Other Comprehensive Income (Loss) (Details) - USD ($)
$ in Millions
 12 Months Ended
Sep. 30, 2025
Sep. 30, 2024
AOCI Attributable to Parent, Net of Tax [Roll Forward]    
Balance at beginning of period $ 185.6 $ 203.2
Other comprehensive loss before reclassification (3.0) (10.4)
(Gain) loss reclassified out of accumulated other comprehensive income (2.1) 4.6
Tax benefit 0.4 2.5
Balance at end of period 338.5 185.6
Unamortized benefit plan credits    
AOCI Attributable to Parent, Net of Tax [Roll Forward]    
Balance at beginning of period 11.7 13.4
Other comprehensive loss before reclassification 0.0 0.0
(Gain) loss reclassified out of accumulated other comprehensive income (2.1) (2.1)
Tax benefit 0.4 0.4
Balance at end of period 10.0 11.7
Currency translation adjustments    
AOCI Attributable to Parent, Net of Tax [Roll Forward]    
Balance at beginning of period (1.8) (6.0)
Other comprehensive loss before reclassification (3.0) (0.4)
(Gain) loss reclassified out of accumulated other comprehensive income 0.0 4.4
Tax benefit 0.0 0.2
Balance at end of period (4.8) (1.8)
Change in fair value of cash flow hedges    
AOCI Attributable to Parent, Net of Tax [Roll Forward]    
Balance at beginning of period 0.0 5.8
Other comprehensive loss before reclassification 0.0 (10.0)
(Gain) loss reclassified out of accumulated other comprehensive income 0.0 2.3
Tax benefit 0.0 1.9
Balance at end of period 0.0 0.0
Accumulated other comprehensive income    
AOCI Attributable to Parent, Net of Tax [Roll Forward]    
Balance at beginning of period 9.9 13.2
Balance at end of period $ 5.2 $ 9.9