Borrowings - Schedule of Components of Interest Expenses (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2026	Jun. 30, 2025	Jun. 30, 2026	Jun. 30, 2025
Debt Disclosure [Abstract]
Stated interest expense	$ 42.5	$ 35.2	$ 128.6	$ 107.6
Credit facility fees	1.9	2.6	5.0	8.2
Amortization of debt financing costs	2.2	2.2	6.8	6.5
Effect of interest rate swaps	(0.7)	0.5	(0.9)	2.6
Total interest expense	$ 45.9	$ 40.5	$ 139.5	$ 124.9
Weighted average interest rate (as a percent)	5.964%	6.915%	6.053%	7.117%
Weighted average outstanding balance	$ 2,774.4	$ 2,042.4	$ 2,783.1	$ 2,027.2