Revenue from Contracts with Customers - Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2019	Mar. 31, 2018
Disaggregation of Revenue [Line Items]
Revenues	$ 326.2	$ 340.7	$ 665.7	$ 671.5
United States
Disaggregation of Revenue [Line Items]
Revenues	96.5	102.4	196.6	197.4
Taiwan
Disaggregation of Revenue [Line Items]
Revenues	59.6	63.3	124.2	122.6
South Korea
Disaggregation of Revenue [Line Items]
Revenues	111.1	112.7	227.1	231.2
China
Disaggregation of Revenue [Line Items]
Revenues	19.4	26.8	38.6	47.8
Europe
Disaggregation of Revenue [Line Items]
Revenues	16.3	15.0	32.0	30.5
Asia, excluding China, Taiwan, and South Korea
Disaggregation of Revenue [Line Items]
Revenues	23.3	20.5	47.2	42.0
Materials \| Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	216.5	218.9	438.2	433.5
Materials \| Operating Segments \| Advanced Materials
Disaggregation of Revenue [Line Items]
Revenues	117.6	119.4	241.2	231.3
Materials \| Operating Segments \| Process Materials
Disaggregation of Revenue [Line Items]
Revenues	98.9	99.5	197.0	202.2
Delivery Systems and Services \| Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	109.1	121.1	226.3	236.4
Delivery Systems and Services \| Operating Segments \| Equipment and Installations
Disaggregation of Revenue [Line Items]
Revenues	92.6	105.1	193.5	205.5
Delivery Systems and Services \| Operating Segments \| Site Services
Disaggregation of Revenue [Line Items]
Revenues	16.5	16.0	32.8	30.9
Corporate \| Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	$ 0.6	$ 0.7	$ 1.2	$ 1.6

Versum Materials, Inc. 10-Q