Segment Reporting - Capital Expenditures (Details) - USD ($) $ in Thousands	3 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Segment Reporting Information [Line Items]
Capital expenditures	$ 10,219	$ 26,366
Operating segment | Electronic Materials
Segment Reporting Information [Line Items]
Capital expenditures	5,434	8,485
Operating segment | Performance Materials
Segment Reporting Information [Line Items]
Capital expenditures	1,310	16,369
Unallocated corporate
Segment Reporting Information [Line Items]
Capital expenditures	$ 3,475	$ 1,512