GOODWILL AND OTHER INTANGIBLE ASSETS - Goodwill Activity (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019
Goodwill [Roll Forward]
Goodwill, beginning balance	$ 710,071	$ 101,083
Foreign currency translation impact	7,371	(3,141)
Goodwill arising from the Acquisition		612,129
Other	1,205
Goodwill, ending balance	718,647	710,071
Electronic Materials
Goodwill [Roll Forward]
Goodwill, beginning balance	352,797	96,083
Foreign currency translation impact	7,628	(3,145)
Goodwill arising from the Acquisition		259,859
Other	0
Goodwill, ending balance	360,425	352,797
Performance Materials
Goodwill [Roll Forward]
Goodwill, beginning balance	357,274	5,000
Foreign currency translation impact	(257)	4
Goodwill arising from the Acquisition		352,270
Other	1,205
Goodwill, ending balance	$ 358,222	$ 357,274