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Pensions - Sensitivity Analysis of Present Value of Defined Benefit Obligations Effected by Changes of Significant Actuarial Assumptions (Detail) - TWD ($)
$ in Thousands
Dec. 31, 2020
Dec. 31, 2019
Actuarial assumption of discount rates [member]    
Disclosure of sensitivity analysis for actuarial assumptions [Line Items]    
Percentage increase in assumption 0.25% 0.25%
Effect on present value of defined benefit obligations due to increase in assumption $ (29,114) $ (27,993)
Percentage decrease in assumption 0.25% 0.25%
Effect on present value of defined benefit obligations due to decrease in assumption $ 30,434 $ 29,284
Actuarial assumption of expected rates of salary increases [member]    
Disclosure of sensitivity analysis for actuarial assumptions [Line Items]    
Percentage increase in assumption 0.25% 0.25%
Effect on present value of defined benefit obligations due to increase in assumption $ 29,471 $ 28,501
Percentage decrease in assumption 0.25% 0.25%
Effect on present value of defined benefit obligations due to decrease in assumption $ (28,365) $ (27,407)