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Financial Instruments - Additional Information (Detail) $ in Thousands, $ in Thousands | 12 Months Ended |
--- | --- | ---
Dec. 31, 2018 USD ($) Customer | Dec. 31, 2018 TWD ($) Customer | Dec. 31, 2017 TWD ($) Customer | Dec. 31, 2016 TWD ($) | Dec. 31, 2018 TWD ($)
Disclosure of detailed information about financial instruments [line items] |  |  |  |  |
Transfers between levels |  | $ 0 | $ 0 |  |
Significant unobservable inputs discount rate | 25.00% | 25.00% |  |  |
Increase percentage of significant unobservable inputs discount rate | 1.00% | 1.00% |  |  |
Fair value of unquoted shares decreased | $ 252 |  |  |  | $ 7,700
Fair value of bonds decreased | $ 229 | $ 7,000 |  |  |
Number of largest customers | Customer | 5 | 5 | 5 |  |
Percentage of trade receivables | 36.00% |  | 33.00% |  | 36.00%
Bottom of range [member] | Actuarial assumption of discount rates [member] |  |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |  |
Significant unobservable inputs discount rate | 20.00% | 20.00% |  |  |
Top of range [member] | Actuarial assumption of discount rates [member] |  |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |  |
Significant unobservable inputs discount rate | 30.00% | 30.00% |  |  |
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 | $ 45,671 | $ 1,398,000 | $ 24,000 | $ 358,000 |
Currency risk [member] |  |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |  |
Value at risk | $ 4,214 |  | 101,000 | 69,000 | $ 129,000
Exchange rate exposure | 100.00% | 100.00% |  |  |
Other price risk [member] |  |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |  |
Increase decrease in profit before tax | $ 2,091 | $ 64,000 | $ 52,000 | $ 26,000 |
Equity debt ratio | 1.00% | 1.00% | 1.00% | 1.00% |
Increase decrease in other comprehensive income | $ 523 | $ 16,000 | $ 13,000 | $ 13,000 |
Decrease in profit before income tax |  |  |  | 510,000 |
Increase in profit before income tax |  |  |  | $ 445,000 |

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