Borrowings - Schedule of Components of Interest and Other Debt Expenses (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2023
2025 Notes
Debt Instrument [Line Items]
Coupon interest	$ 4.2	$ 10.5	$ 10.5
Amortization of financing costs and discount	0.5	1.3	1.3
Effect of interest rate swap	0.0	0.0	0.0
Total interest expense	$ 4.7	$ 11.8	$ 11.8
Coupon interest rate (net of effect of interest rate swaps)	3.50%	3.50%	3.50%
2027 Notes
Debt Instrument [Line Items]
Coupon interest	$ 9.4	$ 9.5	$ 9.5
Amortization of financing costs and discount	0.9	0.9	0.9
Effect of interest rate swap	13.1	16.3	13.4
Total interest expense	$ 23.4	$ 26.7	$ 23.8
Coupon interest rate (net of effect of interest rate swaps)	6.353%	7.259%	6.539%
2029 Notes
Debt Instrument [Line Items]
Coupon interest	$ 21.3	$ 21.3	$ 2.7
Amortization of financing costs and discount	1.3	1.3	0.2
Effect of interest rate swap	1.8	4.4	0.6
Total interest expense	$ 24.4	$ 27.0	$ 3.5
Coupon interest rate (net of effect of interest rate swaps)	7.585%	8.437%	8.49%
2030 Notes
Debt Instrument [Line Items]
Coupon interest	$ 11.3
Amortization of financing costs and discount	0.4
Effect of interest rate swap	0.4
Total interest expense	$ 12.1
Coupon interest rate (net of effect of interest rate swaps)	6.491%