Other Expenses - Other Expenses Breakout (Details) - USD ($) $ in Millions	3 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Other Expenses [Abstract]
Insurance	$ 1.8	$ 1.0
Office supplies and printing	0.3	0.4
Other clearing related expenses	1.0	0.8
Other non-income taxes	3.8	1.3
Contingent consideration, net	0.1	0.0
Other	4.1	2.6
Total other expenses	$ 11.1	$ 6.1