Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Numerator
Net Income Attributable to Versum	$ 52.7	$ 47.8	$ 148.4	$ 166.8
Denominator
Weighted average number of common shares - Basic (shares)	108,800,000	108,700,000	108,700,000	108,700,000
Effect of dilutive securities
Employee stock option and other award plans (shares)	700,000	0	600,000	0
Weighted average number of common shares - Diluted (shares)	109,500,000	108,700,000	109,300,000	108,700,000
Earnings Per Common Share Attributable to Versum
Net Income Attributable to Versum - Basic (usd per share)	$ 0.48	$ 0.44	$ 1.36	$ 1.54
Net Income Attributable to Versum - Diluted (usd per share)	$ 0.48	$ 0.44	$ 1.36	$ 1.54
Share-based awards
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive shares excluded from the computation of earnings per share (shares)	0		0