Summary of Significant Accounting Policies Change in Accounting Principle Financial Impact (Details) - USD ($) $ / shares in Units, $ 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	Sep. 30, 2015
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cost of sales	$ 200.4	$ 200.4	$ 195.9	$ 191.4	$ 171.1	$ 159.6	$ 154.5	$ 151.2	$ 788.1	$ 636.4	$ 542.2
Income tax provision	30.1	19.6	14.2	55.0	11.9	14.4	11.5	15.2	118.9	53.0	58.8
Net Income	56.6	64.1	63.3	20.7	46.6	54.7	46.8	52.1	204.7	200.2	217.2
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
Net income attributable to Versum per common share:
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
Previously Reported
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cost of sales										$ 636.9	$ 539.5
Income tax provision										52.8
Net Income										199.9	219.9
Net Income Attributable to Versum										$ 193.0	$ 212.0
Net income attributable to Versum per common share:
Net Income Attributable to Versum - Basic										$ 1.78	$ 1.95
Net Income Attributable to Versum - Diluted										$ 1.76	$ 1.95
Inventory Costing from LIFO to FIFO | Restatement Adjustment
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cost of sales									$ (0.2)	$ (0.5)	$ 2.7
Income tax provision									0.1	0.2
Net Income										0.3	(2.7)
Net Income Attributable to Versum									$ 0.1	$ 0.3	$ (2.7)	$ 11.0
Net income attributable to Versum per common share:
Net Income Attributable to Versum - Basic										$ 0.00	$ (0.02)
Net Income Attributable to Versum - Diluted										$ 0.01	$ (0.02)