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Income Tax - Items for Which No Deferred Income Tax Assets Have Been Recognized (Detail) - TWD ($) $ in Millions | Dec. 31, 2017 | Dec. 31, 2016
--- | --- | ---
Disclosure of temporary difference, unused tax losses and unused tax credits [Line Items] |  |
Loss carryforwards | $ 341 | $ 335
Deductible temporary differences | 2 | 3
2018 [Member] |  |
Disclosure of temporary difference, unused tax losses and unused tax credits [Line Items] |  |
Loss carryforwards | 126 | 126
2019 [Member] |  |
Disclosure of temporary difference, unused tax losses and unused tax credits [Line Items] |  |
Loss carryforwards | 138 | 138
2020 [Member] |  |
Disclosure of temporary difference, unused tax losses and unused tax credits [Line Items] |  |
Loss carryforwards | 42 | 42
2021 [Member] |  |
Disclosure of temporary difference, unused tax losses and unused tax credits [Line Items] |  |
Loss carryforwards | 9 | 13
2022 [Member] |  |
Disclosure of temporary difference, unused tax losses and unused tax credits [Line Items] |  |
Loss carryforwards | 11 | 1
2023 [Member] |  |
Disclosure of temporary difference, unused tax losses and unused tax credits [Line Items] |  |
Loss carryforwards | 0 | 1
2024 [Member] |  |
Disclosure of temporary difference, unused tax losses and unused tax credits [Line Items] |  |
Loss carryforwards | 0 | 0
2025 [Member] |  |
Disclosure of temporary difference, unused tax losses and unused tax credits [Line Items] |  |
Loss carryforwards | 13 | 14
2026 [Member] |  |
Disclosure of temporary difference, unused tax losses and unused tax credits [Line Items] |  |
Loss carryforwards | 0 | 0
2027 [Member] |  |
Disclosure of temporary difference, unused tax losses and unused tax credits [Line Items] |  |
Loss carryforwards | $ 2 | $ 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 2016-01-13 -Paragraph 81 -Subparagraph e -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=12&date=2016-03-30&anchor=para_81_e&doctype=Standard -URIDate 2016-03-31 + Details
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