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Capital stock - Additional Information (Detail) $ / shares in Units, $ in Thousands, $ in Thousands |  |  | 12 Months Ended |  |  |
--- | --- | --- | --- | --- | --- | ---
Mar. 07, 2019 TWD ($) | Aug. 15, 2018 TWD ($) shares | Dec. 31, 2020 TWD ($) $ / shares shares | Dec. 31, 2018 shares | Dec. 31, 2020 USD ($) shares | Dec. 31, 2019 TWD ($) | Aug. 06, 2019 shares
Disclosure of classes of share capital [line items] |  |  |  |  |  |  |
Ordinary shares par value | $ / shares |  |  | $ 10 |  |  |  |
Number of shares outstanding |  |  | 727,240,000 |  | 727,240,000 |  |
Percentage of capital reduction |  | 15.00% |  |  |  |  |
Amount of capital reduction | $ |  | $ 1,329,446 |  |  |  |  |
Authorised capital | $ |  |  | $ 9,700,000 |  |  |  |
Number of shares authorised |  |  | 970,000,000 |  | 970,000,000 |  |
Paid in capital |  |  | $ 7,272,401 |  | $ 258,989 | $ 7,272,401 |
Approval for number of treasury stock cancelled |  |  |  |  |  |  | 25,570,000
Cancellation of treasury stock | $ | $ 962,503 |  |  |  |  |  |
Top of range [member] |  |  |  |  |  |  |
Disclosure of classes of share capital [line items] |  |  |  |  |  |  |
Share buyback as a percentage of the companies issued shares |  |  | 10.00% |  | 10.00% | 10.00% |
Ordinary shares [member] |  |  |  |  |  |  |
Disclosure of classes of share capital [line items] |  |  |  |  |  |  |
Number of shares outstanding |  |  | 85,412,000 |  | 85,412,000 |  |
Capital reduction |  | 132,945,000 |  | (128,422,000) |  |  |
American depositary shares [member] |  |  |  |  |  |  |
Disclosure of classes of share capital [line items] |  |  |  |  |  |  |
Number of shares represented by each unit of ADS |  |  | 20 |  |  |  |
Number of shares outstanding |  |  | 4,270,600 |  | 4,270,600 |  |

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