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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, 2020 TWD ($) | Dec. 31, 2020 USD ($) | Dec. 31, 2019 TWD ($) | Dec. 31, 2018 TWD ($)
Current income tax: |  |  |  |
Current income tax on profits for the period | $ 636,876 |  | $ 408,788 | $ 326,057
Income tax on unappropriated retained earnings | 105,665 |  | 74,540 | 28,165
Prior year income tax under (over) estimation | (133,923) |  | (5,016) | 3,729
Total current income tax | 608,618 |  | 478,312 | 357,951
Deferred income tax: |  |  |  |
Relating to origination and reversal of temporary differences | (14,237) |  | 35,367 | 101,441
Impact of change in tax rate |  |  |  | (2,774)
Total deferred income tax | (14,237) |  | 35,367 | 98,667
Income tax expense | $ 594,381 | $ 21,167 | $ 513,679 | $ 456,618

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- Definition Adjustments of tax expense (income) recognised in the period for current tax of prior periods. + References Reference 1: http://www.xbrl.org/2003/role/exampleRef -Name IAS -Number 12 -IssueDate 2020-01-01 -Paragraph 80 -Subparagraph b -URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=12&code=ifrs-tx-2020-en-r&anchor=para_80_b&doctype=Standard -URIDate 2020-01-01 + Details
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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 2020-01-01 -Paragraph 80 -Subparagraph a -URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=12&code=ifrs-tx-2020-en-r&anchor=para_80_a&doctype=Standard -URIDate 2020-01-01 + 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 2020-01-01 -Paragraph 81 -Subparagraph g -Clause ii -URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=12&code=ifrs-tx-2020-en-r&anchor=para_81_g_ii&doctype=Standard -URIDate 2020-01-01 + Details
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