Income Taxes - Schedule of Components of Provision for (Benefit from) Income Taxes (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Current:
Federal	$ 70.1	$ 140.9	$ 92.0
Foreign	8.2	(0.2)	22.0
State	12.1	20.2	12.9
Total current	90.4	160.9	126.9
Deferred:
Federal	14.3	2.1	5.4
Foreign	9.7	7.3	(5.5)
State	(1.6)	1.0	(3.0)
Total deferred	22.4	10.4	(3.1)
Provision for income taxes	112.8	171.3	123.8
Allocated to Other Comprehensive Income (Loss)
Deferred federal, state and foreign	$ 8.8	$ (14.9)	$ 11.0