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Income Tax Expense - Major Components of Income Tax Expense (Detail)
$ in Thousands, $ in Thousands
 12 Months Ended
Dec. 31, 2020
TWD ($)
Dec. 31, 2020
USD ($)
Dec. 31, 2019
TWD ($)
Dec. 31, 2018
TWD ($)
Current income tax:        
Current income tax on profits for the period $ 636,876   $ 408,788 $ 326,057
Income tax on unappropriated retained earnings 105,665   74,540 28,165
Prior year income tax under (over) estimation (133,923)   (5,016) 3,729
Total current income tax 608,618   478,312 357,951
Deferred income tax:        
Relating to origination and reversal of temporary differences (14,237)   35,367 101,441
Impact of change in tax rate       (2,774)
Total deferred income tax (14,237)   35,367 98,667
Income tax expense $ 594,381 $ 21,167 $ 513,679 $ 456,618