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Retirement Benefit Plans - Summary of Sensitivity Analysis on Defined Obligations (Detail)
$ in Thousands, $ in Thousands
Dec. 31, 2019
TWD ($)
Dec. 31, 2019
USD ($)
Dec. 31, 2018
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 $ (555,266) $ (18,565) $ (555,181)
Change in defined obligation due to decrease in actuarial assumption 601,616 20,114 603,089
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 591,915 19,790 591,712
Change in defined obligation due to decrease in actuarial assumption $ (545,528) $ (18,239) $ (547,522)