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Net Income - Other Gains and Losses (Detail) $ in Millions, $ in Millions | 12 Months Ended
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Dec. 31, 2020 TWD ($) | Dec. 31, 2020 USD ($) | Dec. 31, 2019 TWD ($) | Dec. 31, 2018 TWD ($)
Analysis Of Income And Expense [Abstract] |  |  |  |
Foreign currency exchange gain or loss, net | $ (47) |  | $ 16 | $ 37
Gain or loss on disposal of investments accounted for using equity method | 10 | $ 0 | 151 | 0
Gain (loss) on disposal of financial instruments | (2) | 0 | 4 | 6
Valuation gain or loss on financial assets and liabilities at fair value through profit or loss, net | (99) | (4) | (38) | (21)
Others | (21) |  | (49) | (68)
Other gains and losses | $ (159) | $ (6) | $ 84 | $ (46)

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- Definition Gain (loss) on disposal of investments accounted for using equity method, net. + References No definition available. + Details
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- Definition The amount of exchange differences recognised in profit or loss that arise from foreign currency transactions, excluding those arising on financial instruments measured at fair value through profit or loss in accordance with IFRS 9. [Refer: At fair value [member]; Financial instruments, class [member]] + References Reference 1: http://www.xbrl.org/2003/role/exampleRef -Name IAS -Number 7 -IssueDate 2020-01-01 -Section A Statement of cash flows for an entity other than a financial institution -URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=7&code=ifrs-tx-2020-en-r&doctype=Illustrative%20Examples&dita_xref=IAS07_IE_A_TI -URIDate 2020-01-01 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Name IAS -Number 21 -IssueDate 2020-01-01 -Paragraph 52 -Subparagraph a -URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=21&code=ifrs-tx-2020-en-r&anchor=para_52_a&doctype=Standard -URIDate 2020-01-01 + Details
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