Segment Reporting - Reconciliation from Segment Adjusted EBITDA (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2019
Segment Reporting Information [Line Items]
Net (loss) income	$ (68,577)	$ 142,828	$ 39,215
Interest expense, net	38,360	41,840	43,335
Provision for income taxes	13,783	30,519	23,891
Depreciation and amortization	132,170	127,737	98,592
EBITDA	115,736	342,924	205,033
Impairment charges	230,392	2,314	67,372
Acquisition and integration-related expense	10,115	10,852	34,709
Environmental accrual	2,508	0	0
Costs related to KMG-Bernuth warehouse fire, net of insurance recovery	(1,050)	1,083	9,905
Costs related to the Pandemic, net of grants received	489	849	0
Charge for fair value write-up of acquired inventory sold	0	0	14,869
Net costs related to restructuring of the wood treatment business	123	(221)	1,530
Adjusted EBITDA	358,313	357,801	333,418
Operating segments | Electronic Materials
Segment Reporting Information [Line Items]
Adjusted EBITDA	323,827	299,037	294,902
Operating segments | Performance Materials
Segment Reporting Information [Line Items]
Adjusted EBITDA	87,961	106,797	91,372
Corporate
Segment Reporting Information [Line Items]
Unallocated corporate expenses	$ (53,475)	$ (48,033)	$ (52,856)