Fair Value Information - Financial Instruments Measured at Fair Value on a Recurring Basis (Detail) - TWD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Disclosure of fair value measurement of assets and liabilities [Line Items]
Amount invested in investment entity		$ 5,751
Measured at FVTPL Held for trading	$ 1	1
Fair value on a recurring basis [Member]
Disclosure of fair value measurement of assets and liabilities [Line Items]
Amount invested in investment entity		3,125
Measured at FVTPL Held for trading	1	1
Cash flow hedge-forward exchange contracts, financial liabilities		1
Hedging financial assets	1
Equity investment	6,933
Fair value on a recurring basis [Member] \| Non-listed stocks [Member]
Disclosure of fair value measurement of assets and liabilities [Line Items]
Non-listed stocks	517
Fair value on a recurring basis [Member] \| Level 1 [Member]
Disclosure of fair value measurement of assets and liabilities [Line Items]
Amount invested in investment entity		3,125
Equity investment	2,900
Fair value on a recurring basis [Member] \| Level 2 [Member]
Disclosure of fair value measurement of assets and liabilities [Line Items]
Measured at FVTPL Held for trading	1	1
Cash flow hedge-forward exchange contracts, financial liabilities		$ 1
Hedging financial assets	1
Fair value on a recurring basis [Member] \| Level 3 [Member]
Disclosure of fair value measurement of assets and liabilities [Line Items]
Equity investment	4,033
Fair value on a recurring basis [Member] \| Level 3 [Member] \| Non-listed stocks [Member]
Disclosure of fair value measurement of assets and liabilities [Line Items]
Non-listed stocks	$ 517

Disclosure of fair value measurement of assets and liabilities.

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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
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-IssueDate 2018-01-01
-Paragraph 55
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- Definition

The amount of financial liabilities classified as derivative instruments. [Refer: Financial assets; Derivatives [member]]

+ References

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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 2018-01-01
-Paragraph 11A
-Subparagraph c
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IFRS&num=7&date=2018-03-01&anchor=para_11A_c&doctype=Standard
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Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Name IFRS
-Number 7
-IssueDate 2018-01-01
-Paragraph 8
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-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IFRS&num=7&date=2018-03-01&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

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-Name IFRS
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- Definition

The amount of non-derivative financial assets that are designated as available for sale or are not classified as (a) loans and receivables; (b) held-to-maturity investments; or (c) financial assets at fair value through profit or loss. [Refer: Derivative financial assets; Financial assets at fair value through profit or loss; Held-to-maturity investments]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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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

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