Segment and Geographic Information - Sales by Product Group (Details) - USD ($) $ in Millions	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
Revenue, Major Customer [Line Items]
Sales	$ 350.8	$ 350.0	$ 340.7	$ 330.8	$ 294.5	$ 290.8	$ 270.8	$ 270.8	$ 1,372.3	$ 1,126.9	$ 970.1
Process Materials
Revenue, Major Customer [Line Items]
Sales									408.0	401.8	387.4
Advanced Materials
Revenue, Major Customer [Line Items]
Sales									477.6	427.9	369.3
Equipment and Installations
Revenue, Major Customer [Line Items]
Sales									421.1	235.4	150.8
Site Services
Revenue, Major Customer [Line Items]
Sales									62.6	58.2	62.6
Corp segment
Revenue, Major Customer [Line Items]
Sales									$ 3.0	$ 3.6	$ 0.0