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Retained Earnings - Details of Retained Earnings (Detail) - TWD ($) $ / shares in Units, $ in Thousands | 12 Months Ended
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Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Disclosure of analysis of other comprehensive income by item [line items] |  |  |
Cash dividend, cash distribution per share | $ 2.20 | $ 1.80 | $ 1.20
Legal reserve [member] |  |  |
Disclosure of analysis of other comprehensive income by item [line items] |  |  |
Appropriation of retained earnings | $ 232,611 | $ 258,416 | $ 110,308
Special Reserve [Member] |  |  |
Disclosure of analysis of other comprehensive income by item [line items] |  |  |
Appropriation of retained earnings | (19,802) | 19,802 |
Cash Dividend [Member] |  |  |
Disclosure of analysis of other comprehensive income by item [line items] |  |  |
Appropriation of retained earnings | $ 1,599,928 | $ 1,309,032 | $ 872,718

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