Summary By Quarter (Unaudited) (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2014
Quarterly Financial Information Disclosure [Abstract]
Sales	$ 248.4	$ 242.7	$ 233.5	$ 245.5	$ 232.4	$ 263.0	$ 258.8	$ 255.1	$ 970.1	$ 1,009.3	$ 942.5
Cost of sales	138.7	135.9	132.5	132.4	139.7	153.5	158.6	164.7	539.5	616.5	638.9
Selling and administrative	33.2	27.3	25.7	23.6	26.6	28.4	27.8	26.8	109.8	109.6	109.5
Research and development	11.5	11.3	10.2	10.9	11.7	9.9	10.0	9.1	43.9	40.7	38.4
Business separation, restructuring and cost reduction actions	2.5	1.1	(1.8)	(0.9)	7.1	8.6	1.7	4.2	0.9	21.6	1.3
Other income (expense), net	0.5	0.3	1.0	1.1	0.6	0.5	0.7	(0.7)	2.9	1.1	6.8
Operating Income	63.0	67.4	67.9	80.6	47.9	63.1	61.4	49.6	278.9	222.0	161.2
Equity affiliates’ income	0.0	0.0	0.0	0.2	0.3	0.0	0.4	0.3	0.2	1.0	1.7
Interest expense	0.4	0.0	0.0	0.0	0.0	0.0	0.0	0.1	0.4	0.1	0.3
Income Before Taxes	62.6	67.4	67.9	80.8	48.2	63.1	61.8	49.8	278.7	222.9	162.6
Income tax provision	15.6	17.6	12.2	13.4	3.6	10.8	11.2	6.1	58.8	31.7	31.9
Net Income	47.0	49.8	55.7	67.4	44.6	52.3	50.6	43.7	219.9	191.2	130.7
Less: Net Income Attributable to Non-controlling Interests	1.8	2.0	1.9	2.2	1.4	2.1	1.8	1.8	7.9	7.1	7.1
Net Income Attributable to Versum	$ 45.2	$ 47.8	$ 53.8	$ 65.2	$ 43.2	$ 50.2	$ 48.8	$ 41.9	$ 212.0	$ 184.1	$ 123.6