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Income Tax - Additional Information (Detail) - TWD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Income tax [Line Items] |  |  |  |
Applicable tax rate |  | 17.00% | 17.00% | 17.00%
Creditable ratios for distribution of earnings |  |  | 20.48% |
Unappropriated earnings [Member] |  |  |  |
Income tax [Line Items] |  |  |  |
Applicable tax rate |  | 10.00% |  |
Scenario Plan |  |  |  |
Income tax [Line Items] |  |  |  |
Applicable tax rate | 20.00% |  |  |
Increase in deferred tax assets | $ 480 |  |  |
Increase in deferred tax liabilities | $ 236 |  |  |
Scenario Plan | Unappropriated earnings [Member] |  |  |  |
Income tax [Line Items] |  |  |  |
Applicable tax rate | 5.00% |  |  |
China [Member] |  |  |  |
Income tax [Line Items] |  |  |  |
Applicable tax rate |  | 25.00% | 25.00% | 25.00%

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- Definition The applicable income tax rate. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Name IAS -Number 12 -IssueDate 2016-01-13 -Paragraph 81 -Subparagraph c -Clause ii -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=12&date=2016-03-30&anchor=para_81_c_ii&doctype=Standard -URIDate 2016-03-31 + Details
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