GOODWILL AND OTHER INTANGIBLE ASSETS - Goodwill Rollforward (Details) $ in Thousands	6 Months Ended
	Mar. 31, 2020 USD ($)
Goodwill [Roll Forward]
Goodwill, beginning balance	$ 710,071
Foreign currency translation impact	(1,453)
Other	1,205
Goodwill, ending balance	709,823
Electronic Materials
Goodwill [Roll Forward]
Goodwill, beginning balance	352,797
Foreign currency translation impact	465
Other	0
Goodwill, ending balance	353,262
Performance Materials
Goodwill [Roll Forward]
Goodwill, beginning balance	357,274
Foreign currency translation impact	(1,918)
Other	1,205
Goodwill, ending balance	$ 356,561