Fair Value Measurements - Fair Value of Debt (Details) - USD ($) $ in Millions	Sep. 30, 2023	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	1,006.4	969.0
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	589.8	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	416.6	400.5
Level 2 | Carrying value (a)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt fair value	1,124.4	1,122.9
Unamortized discounts and issuance costs	(10.7)	(12.1)
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	594.5	593.7
Unamortized discounts and issuance costs	(5.5)	(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	529.9	529.2
Unamortized discounts and issuance costs	$ (5.2)	$ (5.8)