Fair Value Measurements - Fair Value of Debt (Details) - USD ($) $ in Millions	Sep. 30, 2022	Sep. 30, 2021
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Unamortized discounts and issuance costs	$ 12.4	$ 13.8
Level 2 | Fair value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt fair value	969.0	1,153.7
Level 2 | Fair value | Senior Notes | Senior Unsecured Notes Due 2030
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt fair value	568.5	622.4
Level 2 | Fair value | Senior Notes | 2031 Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt fair value	400.5	531.3
Level 2 | Carrying value (a)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt fair value	1,122.9	1,121.6
Unamortized discounts and issuance costs	(12.1)	(13.4)
Level 2 | Carrying value (a) | Senior Notes | Senior Unsecured Notes Due 2030
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt fair value	593.7	593.0
Unamortized discounts and issuance costs	(6.3)	(7.0)
Level 2 | Carrying value (a) | Senior Notes | 2031 Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt fair value	529.2	528.6
Unamortized discounts and issuance costs	$ (5.8)	$ (6.4)