Segment Reporting - Revenue from External Customers (Details) - USD ($) $ in Thousands	3 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Segment Reporting Information [Line Items]
Revenue	$ 287,863	$ 283,143
Electronic Materials | Operating segment
Segment Reporting Information [Line Items]
Revenue	236,798	220,721
Electronic Materials | CMP Slurries | Operating segment
Segment Reporting Information [Line Items]
Revenue	134,721	122,326
Electronic Materials | Electronic Chemicals | Operating segment
Segment Reporting Information [Line Items]
Revenue	80,006	77,582
Electronic Materials | CMP Pads | Operating segment
Segment Reporting Information [Line Items]
Revenue	22,071	20,813
Performance Materials | Operating segment
Segment Reporting Information [Line Items]
Revenue	51,065	62,422
Performance Materials | PIM | Operating segment
Segment Reporting Information [Line Items]
Revenue	25,907	45,141
Performance Materials | Wood Treatment | Operating segment
Segment Reporting Information [Line Items]
Revenue	17,323	10,674
Performance Materials | QED | Operating segment
Segment Reporting Information [Line Items]
Revenue	$ 7,835	$ 6,607