Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended								12 Months Ended
	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Numerator
Net Income Attributable to Versum	$ 53.9	$ 63.3	$ 61.6	$ 18.7	$ 45.1	$ 52.7	$ 44.9	$ 50.6	$ 197.5	$ 193.3	$ 209.3
Denominator
Weighted average number of common shares - Basic									108.9	108.7	108.7
Dilutive Securities, Effect on Basic Earnings Per Share [Abstract]
Employee stock option and other award plans									0.9	0.7	0.0
Weighted average number of common shares - Diluted									109.8	109.4	108.7
Earnings Per Common Share Attributable to Versum
Net Income Attributable to Versum - Basic	$ 0.49	$ 0.58	$ 0.57	$ 0.17	$ 0.41	$ 0.48	$ 0.41	$ 0.47	$ 1.81	$ 1.78	$ 1.93
Net Income Attributable to Versum - Diluted	$ 0.49	$ 0.58	$ 0.56	$ 0.17	$ 0.41	$ 0.48	$ 0.41	$ 0.46	$ 1.80	$ 1.77	$ 1.93
Stock Compensation Plan
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount									0.2	0.0