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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, 2019
TWD ($)
Dec. 31, 2019
USD ($)
Dec. 31, 2018
TWD ($)
Dec. 31, 2017
TWD ($)
Income tax relating to components of other comprehensive income [abstract]        
Deferred income tax Related to remeasurement of defined benefit plans $ 74,308 $ 2,484 $ 55,346 $ (51,217)
Unrealized loss on equity instruments at fair value through other comprehensive income (78,124) (2,612) 0  
Effect of tax rate changes     70,755  
Income tax recognized in other comprehensive income $ (3,816) $ (128) $ 126,101 $ (51,217)