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Equity - Summary of Unrealized Gain (Loss) on Financial Assets at FVTOCI (Detail) $ in Thousands, $ in Thousands | 12 Months Ended |
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Dec. 31, 2019 TWD ($) | Dec. 31, 2019 USD ($) | Dec. 31, 2018 TWD ($) | Dec. 31, 2019 USD ($)
Financial assets at fair value through other comprehensive income [line items] |  |  |  |
Beginning balance | $ 1,597,323 |  |  |
Ending balance | 1,770,775 | $ 59,203 | $ 1,597,323 |
Unrealized gain (loss) on financial assets at fair value through other comprehensive income [member] |  |  |  |
Financial assets at fair value through other comprehensive income [line items] |  |  |  |
Balance at January 1 per IAS 39 | (1,015,107) |  | 422,570 | $ (33,939)
Adjustment on initial application of IFRS 9 (Note 3) |  |  | (287,053) |
Beginning balance | (1,015,107) | (33,939) | 135,517 |
Debt instruments | (2,052) | (69) | (63,076) |
Equity instruments | (283,472) | (9,477) | (398,513) |
Share from associates and joint venture accounted for using the equity method | 1,501,689 | 50,207 | (555,271) |
Disposal of equity instruments and transferred cumulative gain to retained earnings |  |  | (1,518) |
Disposal of associates and joint venture accounted for using the equity method |  |  | (133,364) |
Share from associates and joint venture accounted for using the equity method | (404,156) | (13,512) | 1,118 |
Ending balance | $ (203,098) | $ (6,790) | $ (1,015,107) |

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