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Financial Instruments at Fair Value Through Profit or Loss - Outstanding Forward Exchange Contracts Not Designated for Hedge (Detail) - Forward exchange contracts [Member] | 12 Months Ended |  |
--- | --- | --- | ---
Dec. 31, 2017 TWD ($) | Dec. 31, 2016 TWD ($) | Dec. 31, 2017 USD ($) | Dec. 31, 2017 EUR (€) | Dec. 31, 2016 USD ($) | Dec. 31, 2016 EUR (€)
EUR [Member] |  |  |  |  |  |
Disclosure of detailed information about financial instruments [Line Items] |  |  |  |  |  |
Currency | EUR/NT$ | EUR/NT$ |  |  |  |
Maturity Period | 2018.03-06 | 2017.03 |  |  |  |
Contract Amount | $ 69,000,000 | $ 167,000,000 |  | € 2,000,000 |  | € 5,000,000
USD [Member] |  |  |  |  |  |
Disclosure of detailed information about financial instruments [Line Items] |  |  |  |  |  |
Currency | US$/NT$ | US$/NT$ |  |  |  |
Maturity Period | 2018.01 | 2017.01 |  |  |  |
Contract Amount | $ 125,000,000 | $ 55,000,000 | $ 4,000,000 |  | $ 2,000,000 |

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- Definition Aggregate notional amount specified by the derivative(s). Expressed as an absolute value. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Article 12 -Section 13 -Sentence Column B + Details
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