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Income Tax Expense - Reconciliation of Income Tax Expense and Accounting Profit Before Income Tax (Detail)
$ in Thousands, $ in Thousands
 12 Months Ended
Dec. 31, 2020
TWD ($)
Dec. 31, 2020
USD ($)
Dec. 31, 2019
TWD ($)
Dec. 31, 2018
TWD ($)
Reconciliation of income tax expense and accounting profit before income tax [abstract]        
Tax calculated based on profit before tax and statutory tax rate $ 595,258   $ 606,917 $ 356,488
Effects from adjustments based on regulation 26,974   (162,924) 81,042
Temporary differences not recognized as deferred tax assets (4)   (608) (10,951)
Prior year income tax under (over) estimation (133,923)   (5,016) 3,729
Income tax on unappropriated retained earnings 105,665   74,540 28,165
Impact of change in tax rate       (2,774)
Effect of different tax rates in countries in which the Group operates 411   770 919
Income tax expense $ 594,381 $ 21,167 $ 513,679 $ 456,618