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Employee benefit expenses - Additional Information (Detail) - TWD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Disclosure of defined benefit plans [Line Items]      
Accrued Employee Compensation $ 332,080 $ 338,356 $ 199,027
Accrued Directors remuneration $ 16,604 $ 16,918 $ 9,951
Defined benefit pension plan [Member]      
Disclosure of defined benefit plans [Line Items]      
Employees' compensation distribution rate 10.00%    
Director remuneration rate no more than 0.50%