ANNUAL CONSOLIDATED COMPREHENSIVE INCOME STATEMENTS - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Statement of Comprehensive Income [Abstract]
Net Income	$ 204.7	$ 200.2	$ 217.2
Other Comprehensive Income (Loss), net of tax
Translation adjustments	1.1	(4.9)	22.8
Other Comprehensive Income (Loss), Derivatives Qualifying as Hedges, Net of Tax	0.2	0.0
Pension activity, net of tax of $0.3, $0.5, and $0	(1.3)	(2.2)	0.0
Total Other Comprehensive Income (Loss)	0.0	(7.1)	22.8
Comprehensive Income	204.7	193.1	240.0
Net Income Attributable to Non-controlling Interests	7.2	6.9	7.9
Other Comprehensive Income (Loss) Attributable to Non-controlling Interests	(0.2)	1.3	0.0
Other Comprehensive Income (Loss), Net of Tax, Excluding Transfers	0.0	(4.7)	22.8
Comprehensive Income Attributable to Versum	$ 197.7	$ 184.9	$ 232.1