SEGMENT REPORTING - Revenue from External Customers (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Segment Reporting Information [Line Items]
Revenue	$ 274,727	$ 271,882	$ 842,063	$ 759,051
Operating segment | Electronic Materials
Segment Reporting Information [Line Items]
Revenue	220,367	212,123	659,997	615,586
Operating segment | Electronic Materials | CMP Slurries
Segment Reporting Information [Line Items]
Revenue	117,680	108,617	359,828	345,244
Operating segment | Electronic Materials | Electronic Chemicals
Segment Reporting Information [Line Items]
Revenue	78,614	80,103	234,693	198,474
Operating segment | Electronic Materials | CMP Pads
Segment Reporting Information [Line Items]
Revenue	24,073	23,403	65,476	71,868
Operating segment | Performance Materials
Segment Reporting Information [Line Items]
Revenue	$ 54,360	$ 59,759	$ 182,066	$ 143,465