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Income Tax Expense - Major Components of Income Tax Expense (Detail) $ in Thousands, $ in Thousands | 12 Months Ended
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Dec. 31, 2019 TWD ($) | Dec. 31, 2019 USD ($) | Dec. 31, 2018 TWD ($) | Dec. 31, 2017 TWD ($)
Current income tax: |  |  |  |
Current income tax on profits for the period | $ 408,788 |  | $ 326,057 | $ 125,376
Income tax on unappropriated retained earnings | 74,540 |  | 28,165 | 246,684
Prior year income tax under (over) estimation | (5,016) |  | 3,729 | 67,885
Total current income tax | 478,312 |  | 357,951 | 439,945
Deferred income tax: |  |  |  |
Relating to origination and reversal of temporary differences | 35,367 |  | 101,441 | 110,542
Impact of change in tax rate |  |  | (2,774) |
Total deferred income tax | 35,367 |  | 98,667 | 110,542
Income tax expense | $ 513,679 | $ 17,174 | $ 456,618 | $ 550,487

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- Definition The amount of income taxes payable (recoverable) in respect of the taxable profit (tax loss) for a period. + References Reference 1: http://www.xbrl.org/2003/role/exampleRef -Name IAS -Number 12 -IssueDate 2019-01-01 -Paragraph 80 -Subparagraph a -URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=12&code=ifrs-tx-2019-en-r&anchor=para_80_a&doctype=Standard -URIDate 2019-03-27 + Details
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- Definition The amount of tax expense (income) relating to changes in deferred tax liabilities and deferred tax assets. [Refer: Deferred tax assets; Deferred tax liabilities] + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Name IAS -Number 12 -IssueDate 2019-01-01 -Paragraph 81 -Subparagraph g -Clause ii -URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=12&code=ifrs-tx-2019-en-r&anchor=para_81_g_ii&doctype=Standard -URIDate 2019-03-27 + Details
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