Other Expenses - Other Expenses Breakout (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2019
Other Expenses [Abstract]
Insurance	$ 7.1	$ 4.7	$ 3.4
Office supplies and printing	1.3	1.8	1.9
Other clearing related expenses	4.0	2.8	2.5
Other non-income taxes	14.8	6.6	4.6
Contingent consideration, net	1.1	0.5	0.0
Other	18.0	13.2	10.8
Total other expenses	$ 46.3	$ 29.6	$ 23.2