Summary of Significant Accounting Policies (Details) - USD ($)	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
Accounting Policies [Abstract]
Before-Tax Share-Based Compensation Award Cost									$ 10,100,000	$ 8,300,000	$ 5,000,000
Schedule of Equity Method Investments [Line Items]
Net Income Attributable to Versum	$ 53,900,000	$ 63,300,000	$ 61,600,000	$ 18,700,000	$ 45,100,000	$ 52,700,000	$ 44,900,000	$ 50,600,000	197,500,000	193,300,000	209,300,000
Inventories	177,100,000				160,400,000				177,100,000	160,400,000
Stockholders' equity	212,400,000				30,200,000				212,400,000	30,200,000	(95,100,000)	$ 751,700,000
Cost of sales	200,400,000	200,400,000	195,900,000	191,400,000	171,100,000	159,600,000	154,500,000	151,200,000	788,100,000	636,400,000	542,200,000
Income tax provision	30,100,000	$ 19,600,000	$ 14,200,000	$ 55,000,000	11,900,000	$ 14,400,000	$ 11,500,000	$ 15,200,000	118,900,000	53,000,000	58,800,000
Allowance for doubtful accounts	1,100,000				$ 1,000,000				1,100,000	$ 1,000,000
Residual value of intangible assets	$ 0								$ 0
Daido Air Products Electronics, Inc.
Schedule of Equity Method Investments [Line Items]
Equity method investment ownership, percentage					20.00%					20.00%
Versum Materials
Schedule of Equity Method Investments [Line Items]
Ownership in noncontrolling interest, percentage	74.00%								74.00%
Stockholders' equity	$ 178,500,000				$ (4,400,000)				$ 178,500,000	$ (4,400,000)	(129,000,000)	719,700,000
Inventory Costing from LIFO to FIFO | Restatement Adjustment
Schedule of Equity Method Investments [Line Items]
Net Income Attributable to Versum									100,000	300,000	(2,700,000)	$ 11,000,000
Inventories					8,800,000					8,800,000
Deferred tax liabilities					3,200,000					3,200,000
Stockholders' equity					$ 5,600,000					5,600,000
Cost of sales									(200,000)	(500,000)	$ 2,700,000
Income tax provision									$ 100,000	$ 200,000