Income Taxes - Unrecognized Benefits (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2019
Income Tax Disclosure [Abstract]
Unrecognized tax benefits, beginning balance	$ 1.5	$ 0.0	$ 0.5
Unrecognized tax benefits, increase resulting from current period tax positions	0.0	1.5	0.0
Unrecognized tax benefits, decrease resulting from prior period tax positions	0.0	0.0	(0.5)
Unrecognized tax benefits, ending balance	$ 1.5	$ 1.5	$ 0.0