Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015
Numerator
Net Income Attributable to Versum	$ 44.6	$ 52.7	$ 44.9	$ 50.8	$ 45.2	$ 47.8	$ 53.8	$ 65.2	$ 193.0	$ 212.0	$ 184.1
Denominator
Weighted average number of common shares - Basic									108,700,000	108,700,000	108,700,000
Dilutive Securities, Effect on Basic Earnings Per Share [Abstract]
Employee stock option and other award plans									700,000	0	0
Weighted average number of common shares - Diluted									109,400,000	108,700,000	108,700,000
Earnings Per Common Share Attributable to Versum
Net Income Attributable to Versum - Basic									$ 1.78	$ 1.95	$ 1.69
Net Income Attributable to Versum - Diluted									$ 1.76	$ 1.95	$ 1.69
Stock Compensation Plan
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount									0