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Capital Surplus and Retained Earnings - Details of Retained Earnings (Detail) - TWD ($) $ / shares in Units, $ in Thousands | 12 Months Ended
--- | ---
Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Disclosure of reserves within equity [abstract] |  |  |
Legal reserve | $ 110,308 | $ 302,653 | $ 28,680
Cash dividend | $ 872,718 | $ 256,806 | $ 257,026
Cash dividend, cash distribution per share | $ 1.20 | $ 0.30 | $ 0.30
Cash distribution from capital surplus |  |  | $ 599,728
Cash distribution from capital surplus, cash distribution per share |  |  | $ 0.70

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