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Income Tax - Reconciliation of Accounting Profit and Income Tax Expense (Detail)
$ in Millions, $ in Millions
 12 Months Ended
Dec. 31, 2020
TWD ($)
Dec. 31, 2020
USD ($)
Dec. 31, 2019
TWD ($)
Dec. 31, 2018
TWD ($)
Reconciliation Of Accounting Profit Multiplied By Applicable Tax Rates [Abstract]        
Income before income tax $ 42,826 $ 1,525 $ 41,867 $ 44,986
Income tax expense calculated at the statutory rate 8,565   8,373 8,997
Nondeductible income and expenses in determining taxable income 15   18 227
Unrecognized deductible temporary differences (5)   3 1
Unrecognized loss carryforwards 4   7 21
Tax-exempt income (367)   (148) (580)
Additional income tax under Alternative Minimum Tax Act 0   0 46
Income tax on unappropriated earnings 8   (20) (2,070)
Investment credits (131)   (203) (204)
Change in tax rate 0   0 (38)
Effect of different tax rates of group entities operating in other jurisdictions 10   (9) (15)
Income tax adjustments on prior years 4   (92) 26
Others 19   17 (6)
Income tax expense recognized in profit or loss $ 8,122 $ 289 $ 7,946 $ 6,405