SEGMENT REPORTING - Reconciliation from Segment Adjusted EBITDA (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Segment Reporting Information [Line Items]
Interest expense	$ (10,406)	$ (12,757)	$ (33,079)	$ (32,978)
Interest income	131	417	589	2,004
Depreciation and amortization	(31,324)	(26,587)	(95,516)	(70,476)
Acquisition and integration-related expenses	(2,735)	(2,910)	(7,785)	(33,108)
Charge for fair value write-up of acquired inventory sold	0	(42)	0	(14,869)
Costs related to KMG-Bernuth warehouse fire, net of insurance recovery	(622)	(4,450)	(1,220)	(4,450)
Net costs related to restructuring of the wood treatment business	293	0	293	0
Costs related to the Pandemic, net of grants received	(112)	0	(349)	0
Income before income taxes	47,266	39,428	136,739	94,248
Revenue	274,727	271,882	842,063	759,051
Operating segment | Electronic Materials
Segment Reporting Information [Line Items]
Adjusted EBITDA	76,855	70,859	227,662	220,594
Revenue	220,367	212,123	659,997	615,586
Operating segment | Performance Materials
Segment Reporting Information [Line Items]
Adjusted EBITDA	26,959	27,425	84,370	63,152
Revenue	54,360	59,759	182,066	143,465
Unallocated corporate expenses
Segment Reporting Information [Line Items]
Unallocated corporate expenses	$ (11,773)	$ (12,527)	$ (38,226)	$ (35,621)