Other Expenses - Other Expenses Breakout (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2020	Mar. 31, 2019
Other Expenses [Abstract]
Accretion Expense	$ 0.1	$ 0.0	$ 0.1	$ 0.0
Insurance	1.0	0.9	2.0	1.7
Advertising, meetings, and conferences	4.2	1.5	5.6	2.9
Office supplies and printing	0.5	0.5	0.9	1.0
Other clearing related expenses	0.6	0.7	1.4	1.1
Other non-income taxes	1.3	1.2	2.6	2.1
Other	2.9	2.6	5.5	5.1
Total other expenses	$ 10.6	$ 7.4	$ 18.1	$ 13.9