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Financial Risk Management And Fair Values Of Financial Instruments - Summary of Effect of Profit or Loss or of Other Comprehensive Income from Financial Assets Categorized (Detail) - Financial instruments at fair value through other comprehensive income [member] - TWD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2020 | Dec. 31, 2019
Disclosure of significant unobservable inputs used in fair value measurement of equity [line items] |  |
Favorable Changes | $ 5,325 | $ 2,363
Unfavorable Changes In Fair Value | 5,267 | 2,413
Enter prise value EBITDA multiple [member] |  |
Disclosure of significant unobservable inputs used in fair value measurement of equity [line items] |  |
Favorable Changes | 2,153 | 850
Unfavorable Changes In Fair Value | 2,153 | 900
Discount for lack of marketability [member] |  |
Disclosure of significant unobservable inputs used in fair value measurement of equity [line items] |  |
Favorable Changes | 3,142 | 1,460
Unfavorable Changes In Fair Value | 3,084 | 1,460
Foreign unlisted stocks [member] | Price to book ratio multiple [member] |  |
Disclosure of significant unobservable inputs used in fair value measurement of equity [line items] |  |
Favorable Changes | 30 | 53
Unfavorable Changes In Fair Value | $ 30 | $ 53

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