GOODWILL AND OTHER INTANGIBLE ASSETS - Narrative (Details) - USD ($)	3 Months Ended		9 Months Ended		12 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Sep. 30, 2018
Goodwill [Line Items]
Goodwill	$ 708,390,000		$ 708,390,000		$ 101,083,000
Goodwill, acquired			606,621,000
Goodwill, foreign exchange gain (loss)			686,000
Intangible assets acquired					$ 844,800,000
Amortization of intangible assets	16,926,000	$ 1,780,000	43,244,000	$ 5,716,000
Goodwill, impairment	0
Electronic Materials
Goodwill [Line Items]
Goodwill	358,108,000		358,108,000
Performance Materials
Goodwill [Line Items]
Goodwill	$ 350,282,000		$ 350,282,000