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Income Tax - Reconciliation of Accounting Profit and Income Tax Expense (Detail)
$ in Millions, $ in Millions
 12 Months Ended
Dec. 31, 2017
TWD ($)
Dec. 31, 2017
USD ($)
Dec. 31, 2016
TWD ($)
Dec. 31, 2015
TWD ($)
Reconciliation Of Accounting Profit Multiplied By Applicable Tax Rates [Abstract]        
Income before income tax $ 48,009 $ 1,620 $ 49,413 $ 51,953
Income tax expense calculated at the statutory rate (17%) 8,162   8,400 8,832
Nondeductible revenues and expenses in determining taxable income 34   5 28
Imputed income on tax 0   0 0
Unrecognized deductible temporary differences (1)   (9) 11
Unrecognized loss carryforwards 10   12 83
Tax-exempt income (87)   (25) (183)
Income tax on unappropriated earnings (60)   (346) 821
Investment credits (212)   (234) (329)
Effect of different tax rates of group entities operating in other jurisdictions (2)   (8) (94)
Income tax adjustments on prior years 4   (22) (83)
Others 1   14 15
Income tax expense recognized in profit or loss $ 7,849 $ 265 $ 7,787 $ 9,101