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Financial Risk Management and Fair Values of Financial Instruments - Summary of Movements of Level 3 (Detail) - TWD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2020 | Dec. 31, 2019
Disclosure of detailed information about financial instruments [Line Items] |  |
Movement of financial instruments beginning balance | $ 132,846 | $ 185,828
Gains or losses recognized in profit or loss Recorded as non-operating income (expenses) | (670) | (433)
Gains or losses recognized in other comprehensive gains (losses) Recorded as unrealized gains on valuation of financial assets at fair value through other comprehensive income | 140,199 | (52,549)
Movement of financial instruments ending balance | 272,375 | 132,846
Debt instruments [member] |  |
Disclosure of detailed information about financial instruments [Line Items] |  |
Movement of financial instruments beginning balance | 11,038 | 11,471
Gains or losses recognized in profit or loss Recorded as non-operating income (expenses) | (670) | (433)
Movement of financial instruments ending balance | 10,368 | 11,038
Equity instruments [member] |  |
Disclosure of detailed information about financial instruments [Line Items] |  |
Movement of financial instruments beginning balance | 121,808 | 174,357
Gains or losses recognized in other comprehensive gains (losses) Recorded as unrealized gains on valuation of financial assets at fair value through other comprehensive income | 140,199 | (52,549)
Movement of financial instruments ending balance | $ 262,007 | $ 121,808

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