Other Income (Expense) (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015
Components of other income (expense)
Gain on sale of business	$ 60.8	$ 0.0	$ 0.0
Interest income	19.6	12.7	10.7
Royalty income	8.9	2.9	2.9
Legacy product liability and environmental charges	(8.3)	(12.7)	(19.8)
Other	(0.1)	3.4	0.7
Other income (expense)	80.9	6.3	(5.5)
Proceeds from sale of business	$ 94.0	$ 0.0	$ 0.0