Fair Value Measurements - Fair Value of Debt (Details) - USD ($) $ in Millions	Dec. 31, 2022	Sep. 30, 2022
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Unamortized discounts and issuance costs	$ 12.0	$ 12.4
Level 2 | Fair value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt, fair value	1,019.2	969.0
Level 2 | Fair value | Senior Notes | 2030 Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt, fair value	586.1	568.5
Level 2 | Fair value | Senior Notes | 2031 Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt, fair value	433.1	400.5
Level 2 | Carrying value (a)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt, fair value	1,123.3	1,122.9
Unamortized discounts and issuance costs	(11.7)	(12.1)
Level 2 | Carrying value (a) | Senior Notes | 2030 Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt, fair value	593.9	593.7
Unamortized discounts and issuance costs	(6.1)	(6.3)
Level 2 | Carrying value (a) | Senior Notes | 2031 Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt, fair value	529.4	529.2
Unamortized discounts and issuance costs	$ (5.6)	$ (5.8)