Segment Reporting - Reconciliation from Segment Adjusted EBITDA (Details) - USD ($) $ in Thousands	3 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Segment Reporting Information [Line Items]
Net income	$ 31,530	$ 38,549
Interest expense	9,608	11,920
Interest income	(23)	(315)
Income taxes	7,546	10,881
Depreciation and amortization	31,891	31,642
EBITDA	80,552	92,677
Asset impairment charges	7,347	0
Acquisition and integration-related expenses	2,369	2,204
Costs related to the Pandemic, net of grants received	1,262	0
Net costs related to restructuring of the wood treatment business	26	0
Costs related to KMG-Bernuth warehouse fire, net of insurance recovery	0	392
Consolidated adjusted EBITDA	91,556	95,273
Wood Treatment
Segment Reporting Information [Line Items]
Income taxes	(606)
Asset impairment charges	7,347	0
Operating segment | Electronic Materials
Segment Reporting Information [Line Items]
Consolidated adjusted EBITDA	80,756	81,198
Operating segment | Performance Materials
Segment Reporting Information [Line Items]
Consolidated adjusted EBITDA	22,975	27,478
Unallocated corporate
Segment Reporting Information [Line Items]
Unallocated corporate expenses	$ (12,175)	$ (13,403)