SEGMENT REPORTING - Revenue from External Customers (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Segment Reporting Information [Line Items]
Revenue	$ 274,207	$ 274,727	$ 284,193	$ 283,143	$ 278,645	$ 271,882	$ 265,391	$ 221,778	$ 1,116,270	$ 1,037,696	$ 590,123
Electronic Materials
Segment Reporting Information [Line Items]
Revenue									882,824	833,051	559,945
Electronic Materials | CMP Slurries
Segment Reporting Information [Line Items]
Revenue									480,617	460,053	476,828
Electronic Materials | Electronic Chemicals
Segment Reporting Information [Line Items]
Revenue									316,253	278,413	0
Electronic Materials | CMP Pads
Segment Reporting Information [Line Items]
Revenue									85,954	94,585	83,117
Performance Materials
Segment Reporting Information [Line Items]
Revenue									233,446	204,645	30,178
Performance Materials | PIM
Segment Reporting Information [Line Items]
Revenue									141,503	140,553	0
Performance Materials | Wood Treatment
Segment Reporting Information [Line Items]
Revenue									62,655	31,898	0
Performance Materials | QED
Segment Reporting Information [Line Items]
Revenue									$ 29,288	$ 32,194	$ 30,178