Materials Technologies Separation (Narrative) (Details) - USD ($) $ in Millions		1 Months Ended	3 Months Ended																12 Months Ended
	Jan. 03, 2017	Sep. 30, 2016	Sep. 30, 2017	[1]	Jun. 30, 2017	[1]	Mar. 31, 2017	[1]	Dec. 31, 2016	[1]	Sep. 30, 2016		Jun. 30, 2016		Mar. 31, 2016		Dec. 31, 2015		Sep. 30, 2017		Sep. 30, 2016		Sep. 30, 2015
Restructuring Cost and Reserve [Line Items]
Business separation costs			$ 0.0		$ 0.0		$ 0.0		$ 30.2		$ 21.7	[1]	$ 9.5	[1]	$ 7.4	[1]	$ 12.0	[1]	$ 30.2	[1]	$ 50.6	[1]	$ 7.5
Business separation additional income tax expense																					51.8
Foreign earnings repatriated																					443.8
Loss on extinguishment of debt											6.9	[2]	$ 0.0	[2]	$ 0.0	[2]	$ 0.0	[2]	0.0		6.9	[2]	16.6
Versum Financing
Restructuring Cost and Reserve [Line Items]
Cash to be received at spin-off		$ 550.0									550.0										550.0
Senior notes		425.0									$ 425.0										425.0
Extinguishment of debt amount																					418.3
Loss on extinguishment of debt		$ 6.9																			6.9
Performance Materials Division (PMD) | Maximum
Restructuring Cost and Reserve [Line Items]
Transition services agreement timeframe not to exceed (in months)	12 months
Materials Technologies
Restructuring Cost and Reserve [Line Items]
Business separation costs																			30.2		50.6		$ 7.5
Electronic Materials Division (EMD)
Restructuring Cost and Reserve [Line Items]
Tax benefit related to changes in tax positions on business separation activities																			$ 5.5
Business separation additional income tax expense																					51.8
Tax expense related to repatriation of foreign earnings																					45.7
Foreign earnings repatriated																					$ 443.8

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

[2] For additional information, see Note 15, Debt.