ANNUAL CONSOLIDATED COMPREHENSIVE INCOME STATEMENTS - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015
Statement of Comprehensive Income [Abstract]
Net Income	$ 199.9	$ 219.9	$ 191.2
Other Comprehensive Income (Loss), net of tax
Translation adjustments, net of tax of $0, $0, and $0.7	(4.9)	22.8	(48.4)
Pension activity, net of tax of $0.5, $0, and $0	(2.2)	0.0	0.0
Total Other Comprehensive Income (Loss)	(7.1)	22.8	(48.4)
Comprehensive Income	192.8	242.7	142.8
Net Income Attributable to Non-controlling Interests	6.9	7.9	7.1
Other Comprehensive Loss Attributable to Non-controlling Interests	1.3	0.0	(2.1)
Comprehensive Income Attributable to Versum	$ 184.6	$ 234.8	$ 137.8