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Significant Events After the Reporting Period - Additional Information (Detail) - TWD ($) $ in Millions | 1 Months Ended | 12 Months Ended
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Feb. 28, 2021 | Dec. 31, 2020
Disclosure Of Nonadjusting Events After Reporting Period [Line Items] |  |
Investments at fair value |  | $ 568
Unrealized gain on investments in equity instruments at fair value through other comprehensive income |  | $ 17
Disposal of China Airlines, Ltd. [Member] |  |
Disclosure Of Nonadjusting Events After Reporting Period [Line Items] |  |
Investments at fair value | $ 2,635 |
Unrealized gain on investments in equity instruments at fair value through other comprehensive income | $ 94 |

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- Definition The amount of other comprehensive income, net of tax, related to gains (losses) from changes in the fair value of investments in equity instruments that the entity has designated at fair value through other comprehensive income. [Refer: Other comprehensive income] + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Name IAS -Number 1 -IssueDate 2020-01-01 -Paragraph 7 -URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=1&code=ifrs-tx-2020-en-r&anchor=para_7&doctype=Standard -URIDate 2020-01-01 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Name IAS -Number 1 -IssueDate 2020-01-01 -Paragraph 91 -Subparagraph a -URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=1&code=ifrs-tx-2020-en-r&anchor=para_91_a&doctype=Standard -URIDate 2020-01-01 + Details
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