Other Expenses - Other Expenses Breakout (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Mar. 31, 2021	Mar. 31, 2020
Other Expenses [Abstract]
Insurance	$ 1.9	$ 1.0	$ 3.7	$ 2.0
Office supplies and printing	0.4	0.5	0.7	0.9
Other clearing related expenses	1.0	0.6	2.0	1.4
Other non-income taxes	3.5	1.3	7.3	2.6
Contingent consideration, net	0.8	0.1	0.9	0.1
Other	4.5	2.9	8.6	5.5
Total other expenses	$ 12.1	$ 6.4	$ 23.2	$ 12.5