Other Expenses - Components (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2021
Other Expenses [Abstract]
Non-income taxes	$ 16.8	$ 13.5	$ 14.8
Insurance	11.1	10.8	7.1
Employee related expenses	10.1	9.6	7.0
Other direct business expenses	14.8	10.0	6.3
Membership fees	3.4	3.3	2.8
Director and public company expenses	2.3	1.8	1.5
Office expenses	1.9	1.7	1.3
Other expenses	6.0	9.9	5.5
Total other expenses	$ 66.4	$ 60.6	$ 46.3