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Significant Assets and Liabilities Denominated in Foreign Currencies - Summary of Realized and Unrealized Foreign Exchange Gain (Loss) (Detail) $ in Thousands, $ in Thousands | 12 Months Ended
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Dec. 31, 2017 TWD ($) | Dec. 31, 2017 USD ($) | Dec. 31, 2016 TWD ($) | Dec. 31, 2015 TWD ($)
Disclosure of detailed information about financial instruments [line items] |  |  |  |
Net Foreign Exchange Gain (Loss) | $ 3,547,811 | $ 119,697 | $ 1,921,695 | $ (830,073)
New Taiwan Dollar [member] |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |
Net Foreign Exchange Gain (Loss) | $ 4,130,243 | $ 139,347 | $ 1,494,044 | $ (695,510)
US Dollar [member] | US dollar equals to New Taiwan dollar [member] |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |
Exchange Rate | US$1=NT$29.76 | US$1=NT$29.76 | US$1=NT$32.25 | US$1=NT$32.825
Net Foreign Exchange Gain (Loss) | $ (244,802) | $ (8,259) | $ 203,258 | $ 136,795
Chinese Yuan [member] | Chinese Yuan equals to New Taiwan dollar [member] |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |
Exchange Rate | CNY1=NT$4.5545 | CNY1=NT$4.5545 | CNY1=NT$4.649 | CNY1=NT$5.0550
Net Foreign Exchange Gain (Loss) | $ (337,630) | $ (11,391) | $ 224,393 | $ (271,358)

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