Other Expenses - Other Expenses Breakout (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Other Expenses [Abstract]
Insurance	$ 4.7	$ 3.4	$ 2.6
Office supplies and printing	1.8	1.9	1.7
Other clearing related expenses	2.8	2.5	2.5
Other non-income taxes	6.6	4.6	4.9
Contingent consideration, net	0.5	0.0	0.0
Other	13.2	10.8	8.4
Total other expenses	$ 29.6	$ 23.2	$ 20.1