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Financial Instruments - Summary of Fair Value of Financial Instruments that are Measured at Fair Value on a Recurring Basis (Detail) $ in Thousands, $ in Thousands | Dec. 31, 2017 TWD ($) | Dec. 31, 2017 USD ($) | Dec. 31, 2016 TWD ($)
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Disclosure of detailed information about financial instruments [line items] |  |  |
Financial assets designated as at FVTPL | $ 100,496 | $ 3,391 | $ 100,583
Financial assets held for trading | 5,122,571 | 172,826 | 2,969,229
Financial liabilities at FVTPL | 677,430 | 22,855 | 1,763,660
Financial assets at fair value through profit or loss | 5,223,067 | 176,217 | 3,069,812
Available-for-sale financial assets | 1,212,165 | 40,896 | 1,295,034
Level 1 of fair value hierarchy [member] |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |
Financial assets at fair value through profit or loss | 5,000,708 | 168,715 | 2,440,018
Available-for-sale financial assets | 303,616 | 10,244 | 390,244
Level 2 of fair value hierarchy [Member] |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |
Financial liabilities at FVTPL | 677,430 | 22,855 | 1,763,660
Financial assets at fair value through profit or loss | 222,359 | 7,502 | 629,794
Level 3 of fair value hierarchy [member] |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |
Available-for-sale financial assets | 908,549 | 30,652 | 904,790
Private-placement convertible bonds [member] |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |
Financial assets designated as at FVTPL | 100,496 | 3,391 | 100,583
Private-placement convertible bonds [member] | Level 2 of fair value hierarchy [Member] |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |
Financial assets designated as at FVTPL | 100,496 | 3,391 | 100,583
Forward exchange contracts [member] |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |
Financial assets held for trading | 61,325 | 2,069 | 462,339
Financial liabilities at FVTPL | 25,323 | 854 | 108,912
Forward exchange contracts [member] | Level 2 of fair value hierarchy [Member] |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |
Financial assets held for trading | 61,325 | 2,069 | 462,339
Financial liabilities at FVTPL | 25,323 | 854 | 108,912
Swap contracts [member] |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |
Financial assets held for trading | 60,538 | 2,042 | 462,339
Financial liabilities at FVTPL | 652,107 | 22,001 | 422,934
Swap contracts [member] | Level 2 of fair value hierarchy [Member] |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |
Financial assets held for trading | 60,538 | 2,042 |
Financial liabilities at FVTPL | 652,107 | 22,001 | 422,934
Quoted shares [member] |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |
Financial assets held for trading | 4,410,732 | 148,810 | 1,855,073
Quoted shares [member] | Level 1 of fair value hierarchy [member] |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |
Financial assets held for trading | 4,410,732 | 148,810 | 1,855,073
Open-end mutual funds [member] |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |
Financial assets held for trading | 589,976 | 19,905 | 584,945
Available-for-sale financial assets | 23,825 | 804 | 243,458
Open-end mutual funds [member] | Level 1 of fair value hierarchy [member] |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |
Financial assets held for trading | 589,976 | 19,905 | 584,945
Available-for-sale financial assets | 23,825 | 804 | 243,458
Unquoted shares [Member] |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |
Available-for-sale financial assets | 662,477 | 22,350 | 631,418
Unquoted shares [Member] | Level 3 of fair value hierarchy [member] |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |
Available-for-sale financial assets | 662,477 | 22,350 | 631,418
Limited partnership [member] |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |
Available-for-sale financial assets | 246,072 | 8,302 | 273,372
Limited partnership [member] | Level 3 of fair value hierarchy [member] |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |
Available-for-sale financial assets | 246,072 | 8,302 | 273,372
Quoted ordinary shares [member] |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |
Available-for-sale financial assets | 279,791 | 9,440 | 146,786
Quoted ordinary shares [member] | Level 1 of fair value hierarchy [member] |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |
Available-for-sale financial assets | $ 279,791 | $ 9,440 | 146,786
Foreign currency option contracts [member] |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |
Financial assets held for trading |  |  | 66,872
Financial liabilities at FVTPL |  |  | 17,924
Foreign currency option contracts [member] | Level 2 of fair value hierarchy [Member] |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |
Financial assets held for trading |  |  | 66,872
Financial liabilities at FVTPL |  |  | 17,924
Conversion option, redemption option and put option of convertible bonds [member] |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |
Financial liabilities at FVTPL |  |  | 1,213,890
Conversion option, redemption option and put option of convertible bonds [member] | Level 2 of fair value hierarchy [Member] |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |
Financial liabilities at FVTPL |  |  | $ 1,213,890

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