Fair Value Measurements - Fair Value of Debt (Details) - USD ($) $ in Millions	Sep. 30, 2024	Sep. 30, 2023
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Unamortized discounts and issuance costs	$ 5.6	$ 12.0
Level 2 | Fair value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt fair value	478.5	1,006.4
Level 2 | Fair value | Senior Notes | 2030 Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt fair value	0.0	589.8
Level 2 | Fair value | Senior Notes | 2031 Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt fair value	478.5	416.6
Level 2 | Carrying value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt fair value	530.4	1,124.4
Unamortized discounts and issuance costs	(4.6)	(10.7)
Level 2 | Carrying value | Senior Notes | 2030 Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt fair value	0.0	594.5
Unamortized discounts and issuance costs	0.0	(5.5)
Level 2 | Carrying value | Senior Notes | 2031 Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt fair value	530.4	529.9
Unamortized discounts and issuance costs	$ (4.6)	$ (5.2)