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Accounts receivable - Additional Information (Detail)
$ in Thousands, $ in Thousands
 12 Months Ended
Dec. 31, 2020
TWD ($)
Dec. 31, 2020
USD ($)
Dec. 31, 2019
TWD ($)
Jan. 01, 2019
TWD ($)
Disclosure of financial assets [Line Items]        
Accounts receivable from contract with customers $ 5,364,156 $ 191,031 $ 4,452,904  
Accounts receivable [member]        
Disclosure of financial assets [Line Items]        
Accounts receivable from contract with customers $ 5,365,776   $ 4,454,255 $ 4,745,693
Bottom of range [member]        
Disclosure of financial assets [Line Items]        
Credit term granted to customers 30 days      
Top of range [member]        
Disclosure of financial assets [Line Items]        
Credit term granted to customers 90 days