Income Taxes - Summary of Income Tax Examinations (Details)	12 Months Ended
	Sep. 30, 2018
United States | Minimum
Income Tax Examination [Line Items]
Open Tax Year	2017
United States | Maximum
Income Tax Examination [Line Items]
Open Tax Year	2018
China | Minimum
Income Tax Examination [Line Items]
Open Tax Year	2008
China | Maximum
Income Tax Examination [Line Items]
Open Tax Year	2018
South Korea | Minimum
Income Tax Examination [Line Items]
Open Tax Year	2010
South Korea | Maximum
Income Tax Examination [Line Items]
Open Tax Year	2018
Taiwan | Minimum
Income Tax Examination [Line Items]
Open Tax Year	2013
Taiwan | Maximum
Income Tax Examination [Line Items]
Open Tax Year	2018