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Financial Instruments at Fair Value Through Profit or Loss - Summary of Outstanding Contracts Not Accounted for Hedge Accounting (Detail) € in Thousands, ₩ in Thousands, ¥ in Thousands, ¥ in Thousands, RM in Thousands, $ in Thousands, $ in Thousands, $ in Thousands | 12 Months Ended |  |
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Dec. 31, 2017 TWD ($) | Dec. 31, 2016 TWD ($) | Dec. 31, 2017 USD ($) | Dec. 31, 2017 CNY (¥) | Dec. 31, 2017 JPY (¥) | Dec. 31, 2017 MYR (RM) | Dec. 31, 2017 SGD ($) | Dec. 31, 2017 EUR (€) | Dec. 31, 2016 USD ($) | Dec. 31, 2016 CNY (¥) | Dec. 31, 2016 JPY (¥) | Dec. 31, 2016 KRW (₩) | Dec. 31, 2016 MYR (RM) | Dec. 31, 2016 SGD ($) | Dec. 31, 2016 EUR (€)
Swap contracts [member] | Sell NT$/buy US$ [member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Notional Amount | $ 53,136,302 | $ 59,797,499 | $ 1,782,400 |  |  |  |  |  | $ 1,871,000 |  |  |  |  |  |
Swap contracts [member] | Sell NT$/buy US$ [member] | Bottom of range [member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Maturity Period | 2018-01 | 2017-01 |  |  |  |  |  |  |  |  |  |  |  |  |
Swap contracts [member] | Sell NT$/buy US$ [member] | Top of range [member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Maturity Period | 2018-12 | 2017-12 |  |  |  |  |  |  |  |  |  |  |  |  |
Swap contracts [member] | Sell US$/buy CNY [member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Maturity Period | 2018-01 | 2017-03 |  |  |  |  |  |  |  |  |  |  |  |  |
Notional Amount |  |  | 52,948 | ¥ 349,800 |  |  |  |  | 49,904 | ¥ 349,800 |  |  |  |  |
Swap contracts [member] | Sell US$/buy JPY [member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Maturity Period |  | 2017-02 |  |  |  |  |  |  |  |  |  |  |  |  |
Notional Amount |  |  | 70,324 |  | ¥ 7,870,000 |  |  |  | 77,153 |  | ¥ 8,600,000 |  |  |  |
Swap contracts [member] | Sell US$/buy JPY [member] | Bottom of range [member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Maturity Period | 2018-02 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Swap contracts [member] | Sell US$/buy JPY [member] | Top of range [member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Maturity Period | 2018-03 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Swap contracts [member] | Sell US$/buy NT$ [member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Maturity Period | 2018-01 | 2017-01 |  |  |  |  |  |  |  |  |  |  |  |  |
Notional Amount | $ 6,505,767 | $ 1,958,908 | 217,300 |  |  |  |  |  | 61,000 |  |  |  |  |  |
Forward exchange contracts [member] | Sell NT$/buy US$ [member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Maturity Period | 2018-01 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Notional Amount | $ 2,389,620 | $ 2,842,330 | 80,000 |  |  |  |  |  | 90,000 |  |  |  |  |  |
Forward exchange contracts [member] | Sell NT$/buy US$ [member] | Bottom of range [member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Maturity Period |  | 2017-01 |  |  |  |  |  |  |  |  |  |  |  |  |
Forward exchange contracts [member] | Sell NT$/buy US$ [member] | Top of range [member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Maturity Period |  | 2017-02 |  |  |  |  |  |  |  |  |  |  |  |  |
Forward exchange contracts [member] | Sell US$/buy CNY [member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Notional Amount |  |  | 125,000 | ¥ 828,858 |  |  |  |  | 70,000 | 484,805 |  |  |  |  |
Forward exchange contracts [member] | Sell US$/buy CNY [member] | Bottom of range [member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Maturity Period | 2018-01 | 2017-01 |  |  |  |  |  |  |  |  |  |  |  |  |
Forward exchange contracts [member] | Sell US$/buy CNY [member] | Top of range [member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Maturity Period | 2018-04 | 2017-02 |  |  |  |  |  |  |  |  |  |  |  |  |
Forward exchange contracts [member] | Sell US$/buy JPY [member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Notional Amount |  |  | 45,517 |  | ¥ 5,111,101 |  |  |  | 43,877 |  | ¥ 5,063,820 |  |  |  |
Forward exchange contracts [member] | Sell US$/buy JPY [member] | Bottom of range [member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Maturity Period | 2018-01 | 2017-01 |  |  |  |  |  |  |  |  |  |  |  |  |
Forward exchange contracts [member] | Sell US$/buy JPY [member] | Top of range [member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Maturity Period | 2018-02 | 2017-02 |  |  |  |  |  |  |  |  |  |  |  |  |
Forward exchange contracts [member] | Sell US$/Buy KRW [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Maturity Period |  | 2017-01 |  |  |  |  |  |  |  |  |  |  |  |  |
Notional Amount |  |  |  |  |  |  |  |  | 35,000 |  |  | ₩ 41,012,700 |  |  |
Forward exchange contracts [member] | Sell US$/Buy MYR [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Notional Amount |  |  | 15,000 |  |  | RM 61,859 |  |  | 19,000 |  |  |  | RM 84,544 |  |
Forward exchange contracts [member] | Sell US$/Buy MYR [Member] | Bottom of range [member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Maturity Period | 2018-01 | 2017-01 |  |  |  |  |  |  |  |  |  |  |  |  |
Forward exchange contracts [member] | Sell US$/Buy MYR [Member] | Top of range [member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Maturity Period | 2018-03 | 2017-02 |  |  |  |  |  |  |  |  |  |  |  |  |
Forward exchange contracts [member] | Sell US$/buy NT$ [member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Maturity Period | 2018-01 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Notional Amount | $ 30,142 | $ 6,099,400 | 1,000 |  |  |  |  |  | 190,000 |  |  |  |  |  |
Forward exchange contracts [member] | Sell US$/buy NT$ [member] | Bottom of range [member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Maturity Period |  | 2017-01 |  |  |  |  |  |  |  |  |  |  |  |  |
Forward exchange contracts [member] | Sell US$/buy NT$ [member] | Top of range [member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Maturity Period |  | 2017-03 |  |  |  |  |  |  |  |  |  |  |  |  |
Forward exchange contracts [member] | Sell US$/Buy SGD [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Notional Amount |  |  | 11,300 |  |  |  | $ 15,305 |  | 12,900 |  |  |  |  | $ 18,080 |
Forward exchange contracts [member] | Sell US$/Buy SGD [Member] | Bottom of range [member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Maturity Period | 2018-01 | 2017-01 |  |  |  |  |  |  |  |  |  |  |  |  |
Forward exchange contracts [member] | Sell US$/Buy SGD [Member] | Top of range [member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Maturity Period | 2018-02 | 2017-03 |  |  |  |  |  |  |  |  |  |  |  |  |
Forward exchange contracts [member] | Sell US$/Buy EUR [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Maturity Period | 2018-01 | 2017-01 |  |  |  |  |  |  |  |  |  |  |  |  |
Notional Amount |  |  | $ 10,674 |  |  |  |  | € 9,000 | 281 |  |  |  |  |  | € 270
Foreign currency option contracts [member] | Buy US$ Call/CNY put [member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Maturity Period |  | 2017-08 |  |  |  |  |  |  |  |  |  |  |  |  |
Notional Amount |  |  |  |  |  |  |  |  | 2,000 | 13,800 |  |  |  |  |
Foreign currency option contracts [member] | Buy US$ Put/CNY call [member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Maturity Period |  | 2017-08 |  |  |  |  |  |  |  |  |  |  |  |  |
Notional Amount |  |  |  |  |  |  |  |  | $ 1,000 | ¥ 6,900 |  |  |  |  |

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