Other Expenses - Other Expenses Breakout (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Other Expenses [Abstract]
Non-income taxes	$ 4.1	$ 4.6	$ 11.0	$ 13.7
Insurance	2.5	2.8	8.2	8.2
Employee related expenses	2.1	2.9	8.0	6.9
Other direct business expenses	3.0	2.5	11.1	6.9
Membership fees	0.9	0.6	2.8	2.4
Director and public company expenses	0.5	0.3	1.5	1.3
Office expenses	0.5	0.4	1.4	1.2
Other expenses	1.8	1.7	6.1	4.0
Total other expenses	$ 15.4	$ 15.8	$ 50.1	$ 44.6