SEGMENT REPORTING - Reconciliation from Segment Adjusted EBITDA (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]
Interest expense	$ (10,753)	$ (13,331)	$ (22,673)	$ (20,221)
Interest income	143	568	458	1,587
Depreciation and amortization	(32,550)	(27,348)	(64,192)	(43,889)
Acquisition and integration-related expenses	(2,285)	(2,904)	(5,050)	(30,198)
Charge for fair value write-up of acquired inventory sold	0	(4,566)	0	(14,827)
Costs related to KMG-Bernuth warehouse fire (See Note 13) and restructuring of wood treatment business	(206)	0	(598)	0
Costs related to the Pandemic	(237)	0	(237)	0
Income before income taxes	40,043	37,937	89,473	54,820
Operating segment \| Electronic Materials
Segment Reporting Information [Line Items]
Adjusted EBITDA	69,603	74,910	150,807	149,735
Operating segment \| Performance Materials
Segment Reporting Information [Line Items]
Adjusted EBITDA	29,932	22,660	57,411	35,727
Unallocated corporate expenses
Segment Reporting Information [Line Items]
Unallocated corporate expenses	$ (13,604)	$ (12,052)	$ (26,453)	$ (23,094)

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Charges Related To COVID-19

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Earnings Before Interest, Taxes, Depreciation and Amortization, Unallocated Corporate Expenses

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The amount of expense recognized in the current period that reflects the allocation of capitalized costs associated with acquisition of business. As a noncash expense, this element is added back to net income when calculating cash provided by or used in operations using the indirect method.

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