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Income Tax - Reconciliation of Accounting Profit and Income Tax Expense (Detail) $ in Millions, $ in Millions | 12 Months Ended
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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

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