Financial Instruments - Fair Value of Financial Instruments Measured at Fair Value on a Recurring Basis (Detail) - TWD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Disclosure of detailed information about financial instruments [line items]
Financial assets at FVTPL, Designated as at FVTPL	$ 569.8	$ 6,451.1
Financial liabilities at FVTPL	26.7	191.1
Financial assets at FVTPL, Held for trading	569.8	153.4
Available-for-sale financial assets	98,248.5	71,891.3
Commercial paper [member]
Disclosure of detailed information about financial instruments [line items]
Available-for-sale financial assets	148.3
Measured at fair value on a recurring basis [member]
Disclosure of detailed information about financial instruments [line items]
Financial assets at FVTPL		6,451.1
Financial liabilities at FVTPL		191.1
Available-for-sale financial assets	93,374.2	67,788.8
Hedging derivative financial assets	34.4
Measured at fair value on a recurring basis [member] \| Commercial paper [member]
Disclosure of detailed information about financial instruments [line items]
Available-for-sale financial assets	148.3
Measured at fair value on a recurring basis [member] \| Level 1 [member]
Disclosure of detailed information about financial instruments [line items]
Available-for-sale financial assets	10,264.1	11,543.7
Hedging derivative financial assets	27.0
Measured at fair value on a recurring basis [member] \| Level 2 [member]
Disclosure of detailed information about financial instruments [line items]
Financial assets at FVTPL		6,451.1
Financial liabilities at FVTPL		191.1
Available-for-sale financial assets	83,110.1	56,245.1
Hedging derivative financial assets	7.4
Measured at fair value on a recurring basis [member] \| Level 2 [member] \| Commercial paper [member]
Disclosure of detailed information about financial instruments [line items]
Available-for-sale financial assets	148.3
Measured at fair value on a recurring basis [member] \| Forward exchange contracts [member]
Disclosure of detailed information about financial instruments [line items]
Financial liabilities at FVTPL, Held for trading	26.7	91.6
Financial liabilities at FVTPL, Designated as at FVTPL		99.5
Financial assets at FVTPL, Held for trading	569.8	142.4
Measured at fair value on a recurring basis [member] \| Forward exchange contracts [member] \| Cash flow hedges [member]
Disclosure of detailed information about financial instruments [line items]
Hedging derivative financial liabilities	15.6
Hedging derivative financial assets	7.4
Measured at fair value on a recurring basis [member] \| Forward exchange contracts [member] \| Level 2 [member]
Disclosure of detailed information about financial instruments [line items]
Financial liabilities at FVTPL, Held for trading	26.7	91.6
Financial liabilities at FVTPL, Designated as at FVTPL		99.5
Financial assets at FVTPL, Held for trading	569.8	142.4
Measured at fair value on a recurring basis [member] \| Forward exchange contracts [member] \| Level 2 [member] \| Cash flow hedges [member]
Disclosure of detailed information about financial instruments [line items]
Hedging derivative financial liabilities	15.6
Hedging derivative financial assets	7.4
Measured at fair value on a recurring basis [member] \| Cross currency swap contracts [member]
Disclosure of detailed information about financial instruments [line items]
Financial assets at FVTPL, Held for trading		11.0
Measured at fair value on a recurring basis [member] \| Cross currency swap contracts [member] \| Level 2 [member]
Disclosure of detailed information about financial instruments [line items]
Financial assets at FVTPL, Held for trading		11.0
Measured at fair value on a recurring basis [member] \| Time deposits [member]
Disclosure of detailed information about financial instruments [line items]
Financial assets at FVTPL, Designated as at FVTPL		6,297.7
Measured at fair value on a recurring basis [member] \| Time deposits [member] \| Level 2 [member]
Disclosure of detailed information about financial instruments [line items]
Financial assets at FVTPL, Designated as at FVTPL		6,297.7
Measured at fair value on a recurring basis [member] \| Corporate bonds [member]
Disclosure of detailed information about financial instruments [line items]
Available-for-sale financial assets	40,165.2	29,999.5
Measured at fair value on a recurring basis [member] \| Corporate bonds [member] \| Level 2 [member]
Disclosure of detailed information about financial instruments [line items]
Available-for-sale financial assets	40,165.2	29,999.5
Measured at fair value on a recurring basis [member] \| Agency bonds/Agency mortgage-backed securities [member]
Disclosure of detailed information about financial instruments [line items]
Available-for-sale financial assets	29,235.4	14,880.5
Measured at fair value on a recurring basis [member] \| Agency bonds/Agency mortgage-backed securities [member] \| Level 2 [member]
Disclosure of detailed information about financial instruments [line items]
Available-for-sale financial assets	29,235.4	14,880.5
Measured at fair value on a recurring basis [member] \| Asset-backed securities [member]
Disclosure of detailed information about financial instruments [line items]
Available-for-sale financial assets	13,459.5	11,254.7
Measured at fair value on a recurring basis [member] \| Asset-backed securities [member] \| Level 2 [member]
Disclosure of detailed information about financial instruments [line items]
Available-for-sale financial assets	13,459.5	11,254.7
Measured at fair value on a recurring basis [member] \| Government bonds [member]
Disclosure of detailed information about financial instruments [line items]
Available-for-sale financial assets	7,817.7	8,457.4
Measured at fair value on a recurring basis [member] \| Government bonds [member] \| Level 1 [member]
Disclosure of detailed information about financial instruments [line items]
Available-for-sale financial assets	7,716.0	8,347.0
Measured at fair value on a recurring basis [member] \| Government bonds [member] \| Level 2 [member]
Disclosure of detailed information about financial instruments [line items]
Available-for-sale financial assets	101.7	110.4
Measured at fair value on a recurring basis [member] \| Publicly traded stocks [member]
Disclosure of detailed information about financial instruments [line items]
Available-for-sale financial assets	2,548.1	3,196.7
Measured at fair value on a recurring basis [member] \| Publicly traded stocks [member] \| Level 1 [member]
Disclosure of detailed information about financial instruments [line items]
Available-for-sale financial assets	2,548.1	3,196.7
Measured at fair value on a recurring basis [member] \| Futures Contract [member] \| Fair value hedges [member]
Disclosure of detailed information about financial instruments [line items]
Hedging derivative financial assets	27.0	5.6
Measured at fair value on a recurring basis [member] \| Futures Contract [member] \| Level 1 [member] \| Fair value hedges [member]
Disclosure of detailed information about financial instruments [line items]
Hedging derivative financial assets	$ 27.0	$ 5.6

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

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
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The amount of financial liabilities classified as derivative instruments. [Refer: Financial assets; Derivatives [member]]

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Line items represent concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes of the table.

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No definition available.

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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
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- Definition

The amount of financial assets at fair value through profit or loss classified as held for trading. A financial asset is classified as held for trading if: (a) it is acquired principally for the purpose of selling 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: At fair value [member]; Financial assets at fair value through profit or loss]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Note Expiry date 2018-01-01
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Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
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- Definition

The amount of financial assets at fair value through profit or loss that were designated as such upon initial recognition or subsequently. [Refer: At fair value [member]; Financial assets at fair value through profit or loss]

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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
-Note Expiry date 2018-01-01
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- Definition

The amount of financial liabilities that meet either of the following conditions: (a) they meet the definition of held for trading; or (b) upon initial recognition they are designated by the entity as at fair value through profit or loss. An entity may use this designation only when permitted by paragraph 4.3.5 of IFRS 9 (embedded derivatives) or when doing so results in more relevant information, because either: (a) it eliminates or significantly reduces a measurement or recognition inconsistency (sometimes referred to as ‘an accounting mismatch’) that would otherwise arise from measuring assets or liabilities or recognising the gains and losses on them on different bases; or (b) a group of financial liabilities or financial assets and financial liabilities is managed and its performance is evaluated on a fair value basis, in accordance with a documented risk management or investment strategy, and information about the group is provided internally on that basis to the entity’s key management personnel (as defined in IAS 24). [Refer: At fair value [member]; Key management personnel of entity or parent [member]; Derivatives [member]; Financial assets; Financial liabilities]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IFRS
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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]

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The amount of financial liabilities at fair value through profit or loss that were designated as such upon initial recognition or subsequently. [Refer: At fair value [member]; Financial liabilities at fair value through profit or loss]

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