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Income Tax - Summary of Income Tax Recognized in Other Comprehensive Income (Detail)
$ in Thousands, $ in Thousands
 12 Months Ended
Dec. 31, 2020
TWD ($)
Dec. 31, 2020
USD ($)
Dec. 31, 2019
TWD ($)
Dec. 31, 2018
TWD ($)
Income tax relating to components of other comprehensive income [abstract]        
Deferred income tax Related to remeasurement of defined benefit plans $ 114,559 $ 4,080 $ 74,308 $ 55,346
Unrealized loss on equity instruments at fair value through other comprehensive income (237,460) (8,457) (78,124)  
Effect of tax rate changes       70,755
Income tax recognized in other comprehensive income $ (122,901) $ (4,377) $ (3,816) $ 126,101