REVENUE FROM CONTRACTS WITH CUSTOMERS, Disaggregation of Revenue (Details) $ in Thousands	3 Months Ended
	Dec. 31, 2018 USD ($) Segment	Dec. 31, 2017 USD ($)
REVENUE FROM CONTRACTS WITH CUSTOMERS [Abstract]
Number of reportable segments | Segment	2
Disaggregation of Revenue [Abstract]
Total revenue	$ 221,778	$ 139,979
Electronic Materials [Member]
Disaggregation of Revenue [Abstract]
Total revenue	190,617	133,314
Electronic Materials [Member] | CMP Slurries [Member]
Disaggregation of Revenue [Abstract]
Total revenue	126,329	114,435
Electronic Materials [Member] | Electronic Chemicals [Member]
Disaggregation of Revenue [Abstract]
Total revenue	39,822	0
Electronic Materials [Member] | CMP Pads [Member]
Disaggregation of Revenue [Abstract]
Total revenue	24,466	18,879
Performance Materials [Member]
Disaggregation of Revenue [Abstract]
Total revenue	$ 31,161	$ 6,665