Leases (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2023
Lessee, Lease, Description [Line Items]
Operating lease right-of-use asset	$ 26.2	$ 5.8
Operating lease liability, current	2.6	2.2
Operating lease, liability, noncurrent	26.9	4.3
Operating lease payments, due year one	3.2
Operating lease payments, due year two	4.1
Operating lease payments, due year three	5.3
Operating lease payments, due year four	5.4
Operating lease payments, due year five	5.5
Operating lease payments, due after year five	34.8
Operating lease payments, total due	58.3
Less: Implied interest	19.2
Less: Tenant improvement allowance	9.6
Operating lease liability	29.5	6.5
Total operating lease expense	3.9	3.4	$ 3.1
Variable lease cost	$ 1.3	$ 1.0	$ 0.9
Operating lease, weighted average remaining lease term	10 years	4 years	4 years
Operating lease, weighted average discount rate, percent	7.00%	4.90%	4.80%
Operating lease payments	$ 2.5	$ 2.5	$ 2.4
Operating lease, right-of-use asset, Balance Sheet location	Other assets	Other assets
Operating lease liability, current, Balance Sheet location	Other current liabilities	Other current liabilities
Operating lease liability, non-current, Balance Sheet location	Other liabilities	Other liabilities
Operating lease liability, Balance Sheet location	Other Liabilities	Other Liabilities
Right-of-Use Asset Obtained in Exchange for Operating Lease Liability	$ 22.4
Minimum
Lessee, Lease, Description [Line Items]
Lessee, operating lease, term of contract	1 year
Maximum
Lessee, Lease, Description [Line Items]
Lessee, operating lease, term of contract	12 years