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Financial Instruments - Additional Information (Detail) $ in Thousands, $ in Thousands | 12 Months Ended |
--- | --- | ---
Dec. 31, 2017 TWD ($) | Dec. 31, 2017 USD ($) | Dec. 31, 2016 TWD ($) | Dec. 31, 2015 TWD ($) | Dec. 31, 2017 USD ($)
Disclosure of detailed information about financial instruments [line items] |  |  |  |  |
Transfers between levels | $ 0 |  | $ 0 |  |
Impairment loss | $ (714,675) | $ (24,112) | $ (888,231) | $ (258,129) |
Floating interest rate [member] |  |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |  |
Increase decrease in basis points | 1.00% | 1.00% | 1.00% | 1.00% |
Increase decrease in profit before tax | $ 24,000 | $ 810 | $ 358,000 | $ 117,000 |
Currency risk [member] |  |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |  |
Value at risk | 101,000 |  | 69,000 | 18,000 | $ 3,408
Other price risk [member] |  |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |  |
Increase decrease in profit before tax | $ 52,000 | $ 1,754 | 26,000 | 7,100 |
Equity debt ratio | 1.00% | 1.00% |  |  |
Increase decrease in other comprehensive income | $ 13,000 | $ 439 | 13,000 | 10,000 |
Decrease in profit before income tax |  |  | 510,000 | 605,000 |
Increase in profit before income tax |  |  | 445,000 | $ 638,000 |
Other Gains and Losses Net [member] |  |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |  |
Impairment loss | $ 50,206 | $ 1,694 | $ 90,000 |  |

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