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Equity - Summary of Unrealized Gain (Loss) on Available-for-sale Financial Assets (Detail) $ in Thousands, $ in Thousands | 12 Months Ended
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Dec. 31, 2018 USD ($) | Dec. 31, 2018 TWD ($) | Dec. 31, 2017 TWD ($) | Dec. 31, 2016 TWD ($)
Balance at January 1 |  | $ 201,310,187 | $ 166,841,165 | $ 165,924,980
Unrealized gain (loss) arising on revaluation of available-for-sale financial assets |  |  | 224,036 | (248,599)
Share from associates and joint venture accounted for using the equity method | $ 4,463 | 136,608 | 264,389 | (871,679)
Balance at December 31 | $ 7,155,436 | 219,027,922 | 201,310,187 | 166,841,165
Reserve of gains and losses on remeasuring available-for-sale financial assets excluding changes in capital surplus [member] |  |  |  |
Balance at January 1 |  | $ 422,570 | (197,314) | 588,119
Unrealized gain (loss) arising on revaluation of available-for-sale financial assets |  |  | 169,585 | (257,240)
Cumulative loss reclassified to profit or loss on impairment of available-for-sale financial assets |  |  | 50,206 |
Cumulative loss (gain) reclassified to profit or loss on disposal of available-for-sale financial assets |  |  | (1,517) | 7,512
Share from associates and joint venture accounted for using the equity method |  |  | 401,610 | (535,705)
Balance at December 31 |  |  | $ 422,570 | $ (197,314)

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