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Related Party Transactions - Summary of Purchases of Materials (Detail) - TWD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
JMC ELECTRONICS CO., LTD. [member]      
Disclosure of transactions between related parties [Line Items]      
Purchases of materials $ 9 $ 132,494 $ 130