Income Taxes - Unrecognized Tax Benefits (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Beginning balance	$ 15.7	$ 12.9
Additions for tax positions for the current year	2.8	2.1
Additions for tax positions of prior years	1.8	1.0
Reductions for tax positions of prior years	(4.9)	(0.2)
Statute of limitations expiration	(1.9)	(0.1)
Ending balance	13.5	15.7
Unrecognized Tax Benefits that Would Impact Effective Tax Rate	13.5	$ 15.7
Unrecognized Tax Benefits, Income Tax Penalties and Interest Accrued	$ 1.3