SEGMENT REPORTING - Revenue from External Customers (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]
Revenue	$ 284,193	$ 265,391	$ 567,336	$ 487,169
Electronic Materials
Segment Reporting Information [Line Items]
Revenue	218,909	212,846	439,630	403,462
CMP Slurries
Segment Reporting Information [Line Items]
Revenue	119,822	110,299	242,148	236,627
Electronic Chemicals
Segment Reporting Information [Line Items]
Revenue	78,497	78,549	156,079	118,371
CMP Pads
Segment Reporting Information [Line Items]
Revenue	20,590	23,998	41,403	48,464
Performance Materials
Segment Reporting Information [Line Items]
Revenue	$ 65,284	$ 52,545	$ 127,706	$ 83,707