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Hedging Financial Instruments - Summary of Hedging Instrument (Details) - Forecast Purchases - forward Exchange Contract [Member] - Cash flow hedges [member] € in Millions, $ in Millions | 12 Months Ended |  |  |
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Dec. 31, 2020 TWD ($) | Dec. 31, 2019 TWD ($) | Dec. 31, 2018 TWD ($) | Dec. 31, 2020 EUR (€) | Dec. 31, 2019 EUR (€) | Dec. 31, 2018 EUR (€)
Disclosure Of Detailed Information About Hedging Instruments [Line Items] |  |  |  |  |  |
Change in Value of Hedged Item Used for Calculating Hedge Ineffectiveness | $ (2) | $ 1 | $ (2) |  |  |
Accumulated Gain or Loss on Hedging Instruments in Other Equity Continuing Hedges | 2 | 0 | 1 |  |  |
Accumulated Gain or Loss on Hedging Instruments in Other Equity Hedge Accounting No Longer Applied | $ 0 | $ 0 | $ 0 |  |  |
Foreign currency risk [Member] |  |  |  |  |  |
Disclosure Of Detailed Information About Hedging Instruments [Line Items] |  |  |  |  |  |
Currency | NT$/EUR | NT$/EUR | NT$/EUR |  |  |
Contract Amount | $ 201 | $ 84 | $ 172 | € 6 | € 2 | € 5
Maturity Period | 2021.03 | 2020.03 | 2019.03 |  |  |
Forward Rate | 34.45 | 33.66 | 34.98 |  |  |
Carrying Amount of Asset | $ 2 | $ 0 | $ 1 |  |  |
Carrying Amount of Liability | 0 | 0 | 0 |  |  |
Change in Fair Values of Hedging Instruments Used for Calculating Hedge Ineffectiveness | $ 2 | $ (1) | $ 2 |  |  |

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- Definition Accumulated gain or loss on hedging instruments in other equity hedge accounting no longer applied. + References No definition available. + Details
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- Definition Description of derivative financial instruments maturity period. + References No definition available. + Details
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- Definition Gain loss on change in value of hedging instrument used as basis for recognising hedge ineffectiveness. + References No definition available. + Details
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- Definition The amount of financial assets classified as derivative instruments. [Refer: Financial assets; Derivatives [member]] + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Name IAS -Number 1 -IssueDate 2020-01-01 -Paragraph 55 -URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=1&code=ifrs-tx-2020-en-r&anchor=para_55&doctype=Standard -URIDate 2020-01-01 + Details
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- Definition The amount of financial liabilities classified as derivative instruments. [Refer: Financial assets; Derivatives [member]] + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Name IAS -Number 1 -IssueDate 2020-01-01 -Paragraph 55 -URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=1&code=ifrs-tx-2020-en-r&anchor=para_55&doctype=Standard -URIDate 2020-01-01 + Details
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- Definition The gain (loss) on the change in fair value of the hedging instrument used as a basis for recognising hedge ineffectiveness. Hedge ineffectiveness is the extent to which the changes in the fair value or the cash flows of the hedging instrument are greater or less than those on the hedged item. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Name IFRS -Number 7 -IssueDate 2020-01-01 -Paragraph 24A -Subparagraph c -URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IFRS&num=7&code=ifrs-tx-2020-en-r&anchor=para_24A_c&doctype=Standard -URIDate 2020-01-01 + Details
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