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Income Tax - Items for Which No Deferred Income Tax Assets Have Been Recognized (Detail) - TWD ($) $ in Millions | Dec. 31, 2018 | Dec. 31, 2017
--- | --- | ---
Disclosure of temporary difference, unused tax losses and unused tax credits [Line Items] |  |
Loss carryforwards | $ 237 | $ 215
Deductible temporary differences | 1 | 2
2019 [Member] |  |
Disclosure of temporary difference, unused tax losses and unused tax credits [Line Items] |  |
Loss carryforwards | 136 | 138
2020 [Member] |  |
Disclosure of temporary difference, unused tax losses and unused tax credits [Line Items] |  |
Loss carryforwards | 41 | 42
2021 [Member] |  |
Disclosure of temporary difference, unused tax losses and unused tax credits [Line Items] |  |
Loss carryforwards | 12 | 9
2022 [Member] |  |
Disclosure of temporary difference, unused tax losses and unused tax credits [Line Items] |  |
Loss carryforwards | 10 | 11
2023 [Member] |  |
Disclosure of temporary difference, unused tax losses and unused tax credits [Line Items] |  |
Loss carryforwards | 9 | 0
2024 [Member] |  |
Disclosure of temporary difference, unused tax losses and unused tax credits [Line Items] |  |
Loss carryforwards | 2 | 0
2025 [Member] |  |
Disclosure of temporary difference, unused tax losses and unused tax credits [Line Items] |  |
Loss carryforwards | 15 | 13
2026 [Member] |  |
Disclosure of temporary difference, unused tax losses and unused tax credits [Line Items] |  |
Loss carryforwards | 8 | 0
2027 [Member] |  |
Disclosure of temporary difference, unused tax losses and unused tax credits [Line Items] |  |
Loss carryforwards | 3 | 2
2028 [Member] |  |
Disclosure of temporary difference, unused tax losses and unused tax credits [Line Items] |  |
Loss carryforwards | $ 1 | $ 0

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- Definition The amount of unused tax losses for which no deferred tax asset is recognised in the statement of financial position. [Refer: Unused tax losses [member]] + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Name IAS -Number 12 -IssueDate 2018-01-01 -Paragraph 81 -Subparagraph e -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=12&date=2018-03-01&anchor=para_81_e&doctype=Standard -URIDate 2018-03-16 + Details
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