Segment Reporting - Revenue from External Customers (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2019
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	$ 1,199,831	$ 1,116,270	$ 1,037,696
Electronic Materials
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	984,712	882,824	833,051
Electronic Materials | CMP slurries
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	546,959	480,617	460,053
Electronic Materials | Electronic chemicals
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	330,761	316,253	278,413
Electronic Materials | CMP pads
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	95,335	85,954	94,585
Electronic Materials | Materials technologies
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	11,657	0	0
Performance Materials
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	215,119	233,446	204,645
Performance Materials | PIM
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	106,810	141,503	140,553
Performance Materials | Wood treatment
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	73,188	62,655	31,898
Performance Materials | QED
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	$ 35,121	$ 29,288	$ 32,194