Income Taxes - Schedule of Reconciliation of Unrecognized Tax Benefits (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Reconciliation of the beginning and ending amount of unrecognized tax benefits
Balance at beginning of year	$ 97.3	$ 33.7	$ 37.2
Additions for tax positions related to current year	46.2	63.3	4.2
Additions for tax positions related to prior years	1.4	5.4	5.4
Reductions for tax positions related to prior years	(61.8)	(0.8)	(7.1)
Settlements	(1.3)	(0.9)	(4.1)
Lapse of statutes of limitations	(2.0)	(3.4)	(1.9)
Balance at end of year	$ 79.8	$ 97.3	$ 33.7