SEGMENT REPORTING - Capital Expenditures (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2020	Mar. 31, 2019
Segment Reporting Information [Line Items]
Capital expenditures	$ 37,977	$ 12,531	$ 64,343	$ 20,416
Operating segment | Electronic Materials
Segment Reporting Information [Line Items]
Capital expenditures	5,094	10,150	13,579	16,549
Operating segment | Performance Materials
Segment Reporting Information [Line Items]
Capital expenditures	29,328	1,363	45,697	1,623
Unallocated corporate expenses
Segment Reporting Information [Line Items]
Capital expenditures	$ 3,555	$ 1,018	$ 5,067	$ 2,244