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Capital stock - Movement of Treasury Stock (Detail) shares in Thousands, $ in Thousands | 12 Months Ended
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Dec. 31, 2018 TWD ($) shares
Capital reduction | $ 1,284,223
Treasury stock [member] |
Treasury stock shares at beginning balance | shares | 30,085
Capital reduction | shares | (4,515)
Treasury stock shares at ending balance | shares | 25,570
Beginning balance | $ 1,007,654
Capital reduction | (45,151)
Ending balance | $ 962,503

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