Segment Reporting - Capital Expenditures (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2019
Segment Reporting Information [Line Items]
Capital expenditures	$ 41,747	$ 122,514	$ 62,196
Operating segments | Electronic Materials
Segment Reporting Information [Line Items]
Capital expenditures	28,272	26,536	40,166
Operating segments | Performance Materials
Segment Reporting Information [Line Items]
Capital expenditures	4,094	84,634	16,367
Corporate
Segment Reporting Information [Line Items]
Capital expenditures	$ 9,381	$ 11,344	$ 5,663