Summary By Quarter (Unaudited) (Details) - USD ($) $ 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
Quarterly Financial Information Disclosure [Abstract]
Sales	$ 294.5	$ 290.8	$ 270.8	$ 270.8	$ 248.4	$ 242.7	$ 233.5	$ 245.5	$ 1,126.9	$ 970.1	$ 1,009.3
Cost of sales	171.9	159.6	154.5	150.9	138.7	135.9	132.5	132.4	636.9	539.5	616.5
Selling and administrative	31.5	34.5	29.5	30.2	33.2	27.3	25.7	23.6	125.7	109.8	109.6
Research and development	12.0	11.9	10.9	10.3	11.5	11.3	10.2	10.9	45.1	43.9	40.7
Business separation, restructuring and cost reduction actions	10.2	6.0	6.1	3.2	2.5	1.1	(1.8)	(0.9)	25.5	0.9	21.6
Other (income) expense, net	(1.2)	(2.2)	(0.1)	(2.9)	(0.5)	(0.3)	(1.0)	(1.1)	(6.4)	(2.9)	(1.1)
Operating Income	70.1	81.0	69.9	79.1	63.0	67.4	67.9	80.6	300.1	278.9	222.0
Equity affiliates’ income	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.2	0.0	0.2	1.0
Interest expense	12.4	11.9	11.6	11.5	0.4	0.0	0.0	0.0	47.4	0.4	0.1
Income Before Taxes	57.7	69.1	58.3	67.6	62.6	67.4	67.9	80.8	252.7	278.7	222.9
Income tax provision	11.6	14.4	11.5	15.3	15.6	17.6	12.2	13.4	52.8	58.8	31.7
Net Income	46.1	54.7	46.8	52.3	47.0	49.8	55.7	67.4	199.9	219.9	191.2
Less: Net Income Attributable to Non-controlling Interests	1.5	2.0	1.9	1.5	1.8	2.0	1.9	2.2	6.9	7.9	7.1
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