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Investments accounted for using equity method - Additional Information (Detail) $ in Thousands |  | 12 Months Ended |
--- | --- | --- | ---
Apr. 08, 2019 USD ($) shares | Dec. 31, 2019 TWD ($) | Dec. 31, 2020 TWD ($)
Disclosure of associates [line items] |  |  |
Cash consideration from disposals of investment in associates |  | $ 1,180,179 |
Gain on disposal of investment in associates |  | 973,609 |
Carrying amount of investment in associates |  | 3,392,910 | $ 3,271,677
Individually Immaterial Associates [member] |  |  |
Disclosure of associates [line items] |  |  |
Carrying amount of investment in associates |  | 249,793 | 250,769
JMC ELECTRONICS CO., LTD. [member] |  |  |
Disclosure of associates [line items] |  |  |
Investment in associates at fair value |  | 807,000 | 454,010
Number of common shares disposed | shares | 9,100,000 |  |
Percentage of ownership interest in associate | 10.00% |  |
Cash consideration from disposals of investment in associates | $ 1,180,179 |  |
Gain on disposal of investment in associates | $ 973,609 |  |
Carrying amount of investment in associates |  | $ 249,793 | $ 250,769

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