Materials Technologies Separation (Narrative) (Details) - USD ($) $ in Millions	1 Months Ended	3 Months Ended									12 Months Ended
	Sep. 30, 2016	Sep. 30, 2018	Sep. 30, 2017	[1]	Jun. 30, 2017	[1]	Mar. 31, 2017	[1]	Dec. 31, 2016	[1]	Sep. 30, 2018		Sep. 30, 2017		Sep. 30, 2016
Restructuring Cost and Reserve [Line Items]
Business separation costs, legal and advisory fees, before tax			$ 0.0		$ 0.0		$ 0.0		$ 32.5		$ 0.0		$ 32.5	[1]	$ 50.6
Pension settlement benefit		$ (43.7)									(48.5)	[2]	(10.5)	[2]	(5.1)	[2]
Additional income tax expense related to the separation															51.8
Foreign earnings repatriated															443.8
Loss on extinguishment of debt											$ 0.0		0.0		6.9
Versum Financing
Restructuring Cost and Reserve [Line Items]
Cash to be received at spin-off	$ 550.0														550.0
Senior notes, aggregate principal amount	425.0														425.0
Extinguishment of debt amount	418.3														418.3
Loss on extinguishment of debt	$ 6.9														6.9
Materials Technologies
Restructuring Cost and Reserve [Line Items]
Net business separation costs													30.2		50.6
Business separation costs, legal and advisory fees, before tax													32.5		50.6
Tax benefit related to changes in tax positions on business separation activities													5.5
Materials Technologies | Other Nonoperating Income (Expense) [Member]
Restructuring Cost and Reserve [Line Items]
Pension settlement benefit													$ 2.3
Electronic Materials Division (EMD)
Restructuring Cost and Reserve [Line Items]
Tax expense related to repatriation of foreign earnings															45.7
Foreign earnings repatriated															$ 443.8

[1] For additional information, refer to Note 4, Materials Technologies Separation.

[2] Fiscal year 2018 includes a loss of $43.7 that primarily resulted from the transfer of certain pension payment obligations to an insurer through the purchase of an irrevocable, nonparticipating group annuity contract during the fourth quarter.