Summary By Quarter (Unaudited) (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
Quarterly Financial Information Disclosure [Abstract]
Revenues	$ 350.8	$ 350.0	$ 340.7	$ 330.8	$ 294.5	$ 290.8	$ 270.8	$ 270.8	$ 1,372.3	$ 1,126.9	$ 970.1
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
Selling and administrative	36.0	35.2	36.6	35.3	31.5	34.5	29.5	30.2	143.1	125.7	109.8
Research and development	12.9	12.4	11.1	12.7	12.0	11.9	10.9	10.3	49.1	45.1	43.9
Business separation, restructuring and cost reduction actions	3.9	6.7	8.2	1.8	10.2	6.0	6.1	3.2	20.6	25.5	0.9
Other (income) expense, net	(1.7)	(0.9)	(0.5)	0.5	(1.2)	(2.2)	(0.1)	(2.9)	(2.6)	(6.4)	(2.9)
Operating Income	99.3	96.2	89.4	89.1	70.9	81.0	69.9	78.8	374.0	300.6	276.2
Equity affiliates’ income									0.0	0.0	0.2
Interest expense	12.6	12.5	11.9	11.3	12.4	11.9	11.6	11.5	48.3	47.4	0.4
Gain (Loss) on Extinguishment of Debt	0.0	0.0	0.0	2.1					2.1	0.0	0.0
Income Before Taxes	86.7	83.7	77.5	75.7	58.5	69.1	58.3	67.3	323.6	253.2	276.0
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
Less: Net Income Attributable to Non-controlling Interests	2.7	0.8	1.7	2.0	1.5	2.0	1.9	1.5	7.2	6.9	7.9
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
Earnings Per Share, Basic	$ 0.49	$ 0.58	$ 0.57	$ 0.17	$ 0.41	$ 0.48	$ 0.41	$ 0.47	$ 1.81	$ 1.78	$ 1.93
Earnings Per Share, Diluted	$ 0.49	$ 0.58	$ 0.56	$ 0.17	$ 0.41	$ 0.48	$ 0.41	$ 0.46	$ 1.80	$ 1.77	$ 1.93