Supplemental Information (Other Income (Expense), Net) (Details) - USD ($) $ in Millions		3 Months Ended				12 Months Ended
		Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Supplemental Information [Abstract]
Technology and royalty income	[1]					$ 22.8	$ 20.8	$ 19.0
Interest income	[2]					0.0	1.5	6.1
Foreign exchange						(3.9)	4.3	(7.2)
Sale of assets and investments	[3]					6.9	24.3	8.8
Contract settlements						2.9	14.3	12.6
Transition service agreements reimbursement	[4]					12.7	38.4	0.0
Other						8.8	17.4	10.1
Other income (expense), net						$ 50.2	121.0	$ 49.4
Gain on land sale		$ 12.2	$ 0.0	$ 0.0	$ 0.0		$ 12.2

[1] Primarily includes related party activity with our equity affiliates.

[2] Beginning in the second quarter of fiscal year 2017, interest income associated with our cash and cash items and short-term investments is reflected on the consolidated income statements in "Other non-operating income (expense), net."

[3] Includes a gain of $12.2 resulting from the sale of a parcel of land during the fourth quarter of fiscal year 2017.

[4] Reflects reimbursement for costs in support of transition services agreements with Versum for EMD and with Evonik for PMD. Refer to Note 4, Materials Technologies Separation, for additional information.