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Fair Value Measurements - Fair Value of Debt (Details) - USD ($)
$ in Millions
Mar. 31, 2026
Sep. 30, 2025
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Unamortized discounts and issuance costs $ (19.3) $ (6.6)
Level 2 | Fair value | Senior Notes | 2031 Notes    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Long-term debt, fair value 485.6 491.0
Level 2 | Carrying Value | Senior Notes | 2031 Notes    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Long-term debt, fair value 531.2 531.0
Unamortized discounts and issuance costs $ (3.8) $ (4.0)