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Hedging Financial Instruments - Outstanding Forward Exchange Contracts (Detail) - 12 months ended Dec. 31, 2017 - Forward exchange contracts [Member] - Cash flow hedges [member] € in Millions, $ in Millions | TWD ($) | EUR (€)
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Disclosure Of Detailed Information About Hedging Instruments [Line Items] |  |
Currency | EUR/NT$ |
Maturity Period | 2018.03-06 |
Contract Amount | $ 142 | € 4

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