SEGMENT REPORTING - Capital Expenditures (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Segment Reporting Information [Line Items]
Capital expenditures	$ 122,514	$ 62,196	$ 22,995
Operating segments | Electronic Materials
Segment Reporting Information [Line Items]
Capital expenditures	26,536	40,166	18,668
Operating segments | Performance Materials
Segment Reporting Information [Line Items]
Capital expenditures	84,634	16,367	409
Unallocated corporate
Segment Reporting Information [Line Items]
Capital expenditures	$ 11,344	$ 5,663	$ 3,918