Income Taxes- Summary of Income Tax Benefit or Expense and Effective Income Tax Rate (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Income Tax Disclosure [Abstract]
(Loss) income before income taxes	$ (2.7)	$ 42.7	$ (47.4)	$ 25.9
Income tax (benefit) expense	$ (0.3)	$ 11.0	$ (11.9)	$ 6.6
Effective income tax rate	11.10%	25.80%	25.10%	25.50%