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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
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

Disclosure of fair value measurement of assets and liabilities.

+ References

No definition available.

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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
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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
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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
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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
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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

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-IssueDate 2020-01-01
-Paragraph 8
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