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Financial Risk Management and Fair Values of Financial Instruments - Summary of Qualitative Information and Sensitivity Analysis of Changes in Significant Unobservable Inputs Under Valuation Model Used in Level Fair Value Measurement (Detail) - TWD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2019 | Dec. 31, 2018
Non derivative equity instruments [member] | Foreign unlisted stocks [member] |  |
Disclosure of significant unobservable inputs used in fair value measurement of equity [line items] |  |
Fair value | $ 121,808 | $ 174,357
Valuation technique | Comparable companies | Comparable companies
Non derivative equity instruments [member] | Foreign unlisted stocks [member] | Price book value multiple [member] |  |
Disclosure of significant unobservable inputs used in fair value measurement of equity [line items] |  |
Range (weighted average method) | 1.22% | 1.19%
Relationship of inputs to fair value | The higher the multiple, the higher the fair value | The higher the multiple, the higher the fair value
Non derivative equity instruments [member] | Foreign unlisted stocks [member] | Enter prise value EBITDA multiple [member] |  |
Disclosure of significant unobservable inputs used in fair value measurement of equity [line items] |  |
Range (weighted average method) | 10.51% | 7.69%
Relationship of inputs to fair value | The higher the multiple, the higher the fair value | The higher the multiple, the higher the fair value
Non derivative equity instruments [member] | Foreign unlisted stocks [member] | Discount for lack of marketability [member] |  |
Disclosure of significant unobservable inputs used in fair value measurement of equity [line items] |  |
Range (weighted average method) | 15.80% | 15.80%
Relationship of inputs to fair value | The higher the discount for lack of marketability, the lower the fair value | The higher the discount for lack of marketability, the lower the fair value
Non derivative debt instruments [member] | Foreign partnership interest [member] |  |
Disclosure of significant unobservable inputs used in fair value measurement of equity [line items] |  |
Fair value | $ 11,038 | $ 11,471
Valuation technique | Discounted cash flow | Discounted cash flow
Non derivative debt instruments [member] | Foreign partnership interest [member] | Discount rate [member] |  |
Disclosure of significant unobservable inputs used in fair value measurement of equity [line items] |  |
Range (weighted average method) | 0.30% | 0.35%
Relationship of inputs to fair value | The lower the discount rate, the higher the fair value | The lower the discount rate, the higher the fair value

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