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Fair Value Measurements - Fair Value of Debt (Details) - USD ($) $ in Millions	Jun. 30, 2023	Sep. 30, 2022	Jun. 30, 2022
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents, amortized cost	$ 526.7	$ 23.4	$ 25.5
Unamortized discounts and issuance costs	12.6	12.4
Level 2 \| Fair value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt, fair value	1,028.7	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	590.1	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	438.6	400.5
Level 2 \| Carrying value (a)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt, fair value	1,124.0	1,122.9
Unamortized discounts and issuance costs	(11.0)	(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	594.3	593.7
Unamortized discounts and issuance costs	(5.7)	(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	529.7	529.2
Unamortized discounts and issuance costs	(5.3)	$ (5.8)
U.S. treasury securities \| Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Amortized cost	424.1
Gross unrealized losses	(0.9)
Fair value	423.2
U.S. treasury securities \| Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents, amortized cost	19.9
Cash and cash equivalents, fair value	$ 19.9