CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY (Unaudited) - USD ($) $ in Millions	Total	Common Stock	Capital in Excess of Par	Retained Earnings (Accumulated Deficit)	Accumulated Other Comprehensive Income (Loss)	Total Versum’s Stockholders’ Equity (Deficit)	Non-controlling Interests
Stockholders' equity, beginning balance at Sep. 30, 2017	$ 30.2	$ 108.8	$ 4.8	$ (99.6)	$ (18.4)	$ (4.4)	$ 34.6
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	20.7			18.7		18.7	2.0
Cash dividend	(5.5)		(4.9)	(0.6)		(5.5)
Issuance of common stock through shared based compensation plans	0.1	0.1				0.1
Other comprehensive income (loss)	19.8				19.1	19.1	0.7
Share-based compensation	0.1		0.1			0.1
Stockholders' equity, ending balance at Dec. 31, 2017	65.4	108.9	0.0	(81.5)	0.7	28.1	37.3
Net Income (Loss) Attributable to Parent	143.6
Stockholders' equity, beginning balance at Sep. 30, 2017	30.2	108.8	4.8	(99.6)	(18.4)	(4.4)	34.6
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	148.1
Net Income (Loss) Attributable to Noncontrolling Interest	4.5
Other comprehensive income (loss)	2.8
Stockholders' equity, ending balance at Jun. 30, 2018	163.0	108.9	2.4	34.3	(15.6)	130.0	33.0
Stockholders' equity, beginning balance at Dec. 31, 2017	65.4	108.9	0.0	(81.5)	0.7	28.1	37.3
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	63.3			61.6		61.6	1.7
Cash dividend	(5.4)		(2.8)	(2.6)		(5.4)
Other comprehensive income (loss)	6.6				5.7	5.7	0.9
Share-based compensation	2.8		2.8			2.8
Stockholders' equity, ending balance at Mar. 31, 2018	132.7	108.9	0.0	(22.5)	6.4	92.8	39.9
Net Income (Loss) Attributable to Parent	63.3
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	64.1			63.3		63.3	0.8
Net Income (Loss) Attributable to Noncontrolling Interest	0.8
Cash dividend	(6.5)			(6.5)		(6.5)
Other comprehensive income (loss)	(23.6)				(22.0)	(22.0)	(1.6)
Share-based compensation	2.4		2.4			2.4
Dividends to non-controlling interests	(6.1)						6.1
Stockholders' equity, ending balance at Jun. 30, 2018	163.0	108.9	2.4	34.3	(15.6)	130.0	33.0
Stockholders' equity, beginning balance at Sep. 30, 2018	212.4	109.0	6.1	81.6	(18.2)	178.5	33.9
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	63.1			61.1		61.1	2.0
Cash dividend	(8.8)			(8.8)		(8.8)
Issuance of common stock through shared based compensation plans	0.1	0.1				0.1
Other comprehensive income (loss)	0.9				0.9	0.9
Share-based compensation	(0.9)		(0.9)			(0.9)
Stockholders' equity, ending balance at Dec. 31, 2018	266.8	109.1	5.2	133.9	(17.3)	230.9	35.9
Net Income (Loss) Attributable to Parent	24.8
Stockholders' equity, beginning balance at Sep. 30, 2018	212.4	109.0	6.1	81.6	(18.2)	178.5	33.9
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	29.0
Net Income (Loss) Attributable to Noncontrolling Interest	4.2
Other comprehensive income (loss)	(11.0)
Stockholders' equity, ending balance at Jun. 30, 2019	203.6	109.3	11.5	80.1	(28.7)	172.2	31.4
Stockholders' equity, beginning balance at Dec. 31, 2018	266.8	109.1	5.2	133.9	(17.3)	230.9	35.9
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	51.1			50.4		50.4	0.7
Cash dividend	(8.8)			(8.8)		(8.8)
Issuance of common stock through shared based compensation plans	0.1	0.1				0.1
Other comprehensive income (loss)	(6.8)				(6.5)	(6.5)	(0.3)
Share-based compensation	3.5		3.5			3.5
Stockholders' equity, ending balance at Mar. 31, 2019	305.9	109.2	8.7	175.5	(23.8)	269.6	36.3
Net Income (Loss) Attributable to Parent	(86.7)			(86.7)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	(85.2)					(86.7)
Net Income (Loss) Attributable to Noncontrolling Interest	1.5						1.5
Cash dividend	(8.7)			(8.7)		(8.7)
Issuance of common stock through shared based compensation plans	0.1	0.1				0.1
Other comprehensive income (loss)	(5.1)				(4.9)	(4.9)	(0.2)
Share-based compensation	2.8		2.8			2.8
Dividends to non-controlling interests	(6.2)						(6.2)
Stockholders' equity, ending balance at Jun. 30, 2019	$ 203.6	$ 109.3	$ 11.5	$ 80.1	$ (28.7)	$ 172.2	$ 31.4