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Retirement Benefit Plans - Summary of Sensitivity Analysis on Defined Obligations (Detail)
$ in Thousands, $ in Thousands
Dec. 31, 2020
TWD ($)
Dec. 31, 2020
USD ($)
Dec. 31, 2019
TWD ($)
Actuarial assumption of discount rates [member]      
Disclosure of sensitivity analysis for actuarial assumptions [line items]      
Change in defined obligation due to increase in actuarial assumption $ (617,803) $ (22,002) $ (555,266)
Change in defined obligation due to decrease in actuarial assumption 600,451 21,384 601,616
Actuarial assumption of expected rates of salary increases [member]      
Disclosure of sensitivity analysis for actuarial assumptions [line items]      
Change in defined obligation due to increase in actuarial assumption 611,217 21,767 591,915
Change in defined obligation due to decrease in actuarial assumption $ (565,555) $ (20,141) $ (545,528)