SEGMENT REPORTING - Reconciliation from Segment Adjusted EBITDA (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Segment Reporting Information [Line Items]
Net income (loss)	$ 36,855	$ 34,525	$ 32,899	$ 38,549	$ (20,243)	$ 18,878	$ 27,137	$ 13,443	$ 142,828	$ 39,215	$ 110,043
Interest expense									42,510	45,681	2,905
Interest income									(670)	(2,346)	(4,409)
Income taxes									30,519	23,891	51,668
Depreciation and amortization									127,737	98,592	25,876
EBITDA									342,924	205,033	186,083
Acquisition and integration-related expense									10,852	34,709	3,861
Charges related to asset impairment of wood treatment business									2,314	67,372	0
Costs related to KMG-Bernuth warehouse fire, net of insurance recovery									1,083	9,905	0
Costs related to the Pandemic, net of grants received									849	0	0
Charge for fair value write-up of acquired inventory sold									0	14,869	0
Net costs related to restructuring of the wood treatment business									(221)	1,530	0
Adjusted EBITDA									357,801	333,418	189,944
Wood Treatment
Segment Reporting Information [Line Items]
Income taxes									(608)	(17,072)
Charges related to asset impairment of wood treatment business									2,314	67,372	0
Operating segments | Electronic Materials
Segment Reporting Information [Line Items]
Adjusted EBITDA									299,037	294,902	222,019
Operating segments | Performance Materials
Segment Reporting Information [Line Items]
Adjusted EBITDA									106,797	91,372	7,191
Unallocated corporate
Segment Reporting Information [Line Items]
Unallocated corporate expenses									$ (48,033)	$ (52,856)	$ (39,266)