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Income Tax - Summary of Reconciliation of Income Tax Expense (Detail)
$ in Thousands, $ in Thousands
 12 Months Ended
Dec. 31, 2020
TWD ($)
Dec. 31, 2020
USD ($)
Dec. 31, 2019
TWD ($)
Dec. 31, 2018
TWD ($)
Major components of tax expense (income) [abstract]        
Profit before income tax $ 35,768,798 $ 1,273,817 $ 23,279,811 $ 31,937,678
Income tax expense calculated at the statutory rates 16,907,904 602,133 11,802,811 13,540,599
Nontaxable expense in determining taxable income 316,619 11,276 459,133 353,019
Tax-exempt income (387,212) (13,790) (495,883) (2,515,453)
Additional income tax on unappropriated earnings 680,649 24,240 19,115 (1,022,560)
Loss carry-forward and income tax credits currently used (1,191,387) (42,428) (898,198) (971,124)
Remeasurement of deferred income tax assets, net (8,650,569) (308,069) (5,588,335) (4,776,271)
Changes in estimate for prior periods (472,512) (16,827) (352,579) (103,822)
Unrecognized deductible temporary differences (138,890) (4,946)    
Withholding tax 52,296 1,862 65,182 8,981
Income tax expense recognized in profit or loss $ 7,116,898 $ 253,451 $ 5,011,246 $ 4,513,369