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Pensions - Additional Information (Detail)
$ in Thousands
 12 Months Ended
Dec. 31, 2020
TWD ($)
Unit
Dec. 31, 2019
TWD ($)
Dec. 31, 2018
TWD ($)
Disclosure of defined benefit plans [Line Items]      
Defined benefit plans expected contribution | $ $ 26,261    
Weighted average duration of defined benefit obligation 12 years 7 months 6 days    
Defined benefit pension plan [Member]      
Disclosure of defined benefit plans [Line Items]      
Number of units accrued in first tranche 2    
Duration of first tranche First 15 years    
Number of units accrued in second tranche 1    
Duration of second tranche Thereafter 15 years    
Maximum number of units accruable under the pension plan 45    
Benefit measurement period 6 months    
Employer contribution percentage 2.00%    
Defined contribution pension plan [member]      
Disclosure of defined benefit plans [Line Items]      
Employer contribution percentage 6.00% 6.00%  
Pension costs under defined contribution pension plans | $ $ 184,562 $ 187,502 $ 193,047