Other Expenses - Other Expenses Breakout (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Other Expenses [Abstract]
Insurance	$ 1.6	$ 1.1	$ 5.3	$ 3.1
Office supplies and printing	0.2	0.6	0.9	1.5
Other clearing related expenses	0.9	0.8	2.9	2.2
Other non-income taxes	4.1	1.2	11.4	3.8
Contingent consideration, net	0.0	0.2	0.9	0.3
Other	3.9	3.2	12.5	8.7
Total other expenses	$ 10.7	$ 7.1	$ 33.9	$ 19.6