Income Taxes - Provisions for Income Taxes (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Current expense (benefit):
Federal	$ 51.7	$ (2.7)	$ (2.7)
State	14.0	26.0	14.0
Foreign	69.6	77.6	56.2
Total current expense	135.3	100.9	67.5
Deferred expense (benefit):
Federal	2.9	(77.1)	125.8
State	3.5	6.7	16.4
Foreign	(48.8)	1.9	(31.8)
Investment tax credit amortization	(0.3)	(0.3)	(0.3)
Total deferred expense	(42.7)	(68.8)	110.1
Total income tax expense	$ 92.6	$ 32.1	$ 177.6