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Fair Value Information - Financial Instruments Measured at Fair Value on a Recurring Basis (Detail) - TWD ($) $ in Millions | Dec. 31, 2020 | Dec. 31, 2019
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Disclosure of fair value measurement of assets and liabilities [Line Items] |  |
Financial liabilities at FVTPL Derivative | $ 0 | $ 0
Fair value on a recurring basis [Member] |  |
Disclosure of fair value measurement of assets and liabilities [Line Items] |  |
Financial assets at FVTPL | 687 | 779
Hedging financial assets | 2 | 0
Financial assets at FVOCI | 7,193 | 7,154
Financial liabilities at FVTPL Derivative | 0 | 0
Fair value on a recurring basis [Member] | Level 1 [Member] |  |
Disclosure of fair value measurement of assets and liabilities [Line Items] |  |
Financial assets at FVTPL | 8 | 1
Hedging financial assets | 0 | 0
Financial assets at FVOCI | 2,754 | 2,454
Financial liabilities at FVTPL Derivative | 0 | 0
Fair value on a recurring basis [Member] | Level 2 [Member] |  |
Disclosure of fair value measurement of assets and liabilities [Line Items] |  |
Financial assets at FVTPL | 2 | 0
Hedging financial assets | 2 | 0
Financial assets at FVOCI | 0 | 0
Financial liabilities at FVTPL Derivative | 0 | 0
Fair value on a recurring basis [Member] | Level 3 [Member] |  |
Disclosure of fair value measurement of assets and liabilities [Line Items] |  |
Financial assets at FVTPL | 677 | 778
Hedging financial assets | 0 | 0
Financial assets at FVOCI | 4,439 | 4,700
Financial liabilities at FVTPL Derivative | 0 | 0
Fair value on a recurring basis [Member] | Non-listed stocks [Member] |  |
Disclosure of fair value measurement of assets and liabilities [Line Items] |  |
Financial assets at FVTPL | 677 | 778
Financial assets at FVOCI | 4,439 | 4,700
Fair value on a recurring basis [Member] | Non-listed stocks [Member] | Level 1 [Member] |  |
Disclosure of fair value measurement of assets and liabilities [Line Items] |  |
Financial assets at FVTPL | 0 | 0
Financial assets at FVOCI | 0 | 0
Fair value on a recurring basis [Member] | Non-listed stocks [Member] | Level 2 [Member] |  |
Disclosure of fair value measurement of assets and liabilities [Line Items] |  |
Financial assets at FVTPL | 0 | 0
Financial assets at FVOCI | 0 | 0
Fair value on a recurring basis [Member] | Non-listed stocks [Member] | Level 3 [Member] |  |
Disclosure of fair value measurement of assets and liabilities [Line Items] |  |
Financial assets at FVTPL | 677 | 778
Financial assets at FVOCI | 4,439 | 4,700
Fair value on a recurring basis [Member] | Derivatives [Member] |  |
Disclosure of fair value measurement of assets and liabilities [Line Items] |  |
Financial assets at FVTPL | 2 | 0
Fair value on a recurring basis [Member] | Derivatives [Member] | Level 1 [Member] |  |
Disclosure of fair value measurement of assets and liabilities [Line Items] |  |
Financial assets at FVTPL | 0 | 0
Fair value on a recurring basis [Member] | Derivatives [Member] | Level 2 [Member] |  |
Disclosure of fair value measurement of assets and liabilities [Line Items] |  |
Financial assets at FVTPL | 2 | 0
Fair value on a recurring basis [Member] | Derivatives [Member] | Level 3 [Member] |  |
Disclosure of fair value measurement of assets and liabilities [Line Items] |  |
Financial assets at FVTPL | 0 | 0
Fair value on a recurring basis [Member] | Listed stocks [Member] |  |
Disclosure of fair value measurement of assets and liabilities [Line Items] |  |
Financial assets at FVTPL | 8 | 1
Financial assets at FVOCI | 2,754 | 2,454
Fair value on a recurring basis [Member] | Listed stocks [Member] | Level 1 [Member] |  |
Disclosure of fair value measurement of assets and liabilities [Line Items] |  |
Financial assets at FVTPL | 8 | 1
Financial assets at FVOCI | 2,754 | 2,454
Fair value on a recurring basis [Member] | Listed stocks [Member] | Level 2 [Member] |  |
Disclosure of fair value measurement of assets and liabilities [Line Items] |  |
Financial assets at FVTPL | 0 | 0
Financial assets at FVOCI | 0 | 0
Fair value on a recurring basis [Member] | Listed stocks [Member] | Level 3 [Member] |  |
Disclosure of fair value measurement of assets and liabilities [Line Items] |  |
Financial assets at FVTPL | 0 | 0
Financial assets at FVOCI | $ 0 | $ 0

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- Definition The amount of derivative financial assets held for hedging. [Refer: Derivative financial assets] + 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 investments in equity instruments that the entity has designated at fair value through other comprehensive income. [Refer: At fair value [member]; Other comprehensive income] + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Name IFRS -Number 7 -IssueDate 2020-01-01 -Paragraph 11A -Subparagraph c -URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IFRS&num=7&code=ifrs-tx-2020-en-r&anchor=para_11A_c&doctype=Standard -URIDate 2020-01-01 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Name IFRS -Number 7 -IssueDate 2020-01-01 -Paragraph 8 -Subparagraph h -URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IFRS&num=7&code=ifrs-tx-2020-en-r&anchor=para_8_h&doctype=Standard -URIDate 2020-01-01 + Details
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- Definition The amount of financial assets that are measured at fair value and for which gains (losses) are recognised in profit or loss. A financial asset shall be measured at fair value through profit or loss unless it is measured at amortised cost or at fair value through other comprehensive income. A gain (loss) on a financial asset measured at fair value shall be recognised in profit or loss unless it is part of a hedging relationship, it is an investment in an equity instrument for which the entity has elected to present gains and losses in other comprehensive income or it is a financial asset measured at fair value through other comprehensive income. [Refer: At fair value [member]; Financial assets] + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Name IFRS -Number 7 -IssueDate 2020-01-01 -Paragraph 8 -Subparagraph a -URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IFRS&num=7&code=ifrs-tx-2020-en-r&anchor=para_8_a&doctype=Standard -URIDate 2020-01-01 + Details
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- Definition The amount of financial liabilities at fair value through profit or loss that meet the definition of held for trading. A financial liability is classified as held for trading if: (a) it is acquired or incurred principally for the purpose of selling or repurchasing it in the near term; (b) on initial recognition it is part of a portfolio of identified financial instruments that are managed together and for which there is evidence of a recent actual pattern of short-term profit-taking; or (c) it is a derivative (except for a derivative that is a financial guarantee contract or a designated and effective hedging instrument). [Refer: Financial liabilities at fair value through profit or loss] + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Name IFRS -Number 7 -IssueDate 2020-01-01 -Paragraph 8 -Subparagraph e -URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IFRS&num=7&code=ifrs-tx-2020-en-r&anchor=para_8_e&doctype=Standard -URIDate 2020-01-01 + Details
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