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Effects on initial application of IFRS 9 - Summary of Carrying Amount of Financial Assets Transferred (Parenthetical) (Detail) $ in Thousands, $ in Thousands | Dec. 31, 2020 TWD ($) | Dec. 31, 2020 USD ($) | Dec. 31, 2019 TWD ($) | Dec. 31, 2018 TWD ($) | Jan. 01, 2018 TWD ($) | Dec. 31, 2017 TWD ($)
--- | --- | --- | --- | --- | --- | ---
Disclosure of financial assets [Line Items] |  |  |  |  |  |
Financial assets at amortized cost | $ 254,801 |  | $ 237,420 |  |  |
Retained earnings | 5,401,569 | $ 192,364 | 4,651,215 |  |  |
Deferred tax liabilities | $ 310,427 | $ 11,055 | $ 309,129 | $ 308,759 |  | $ 174,293
Increase (decrease) due to changes in accounting policy required by IFRSs [member] |  |  |  |  |  |
Disclosure of financial assets [Line Items] |  |  |  |  |  |
Retained earnings |  |  |  |  | $ (1,940) |
IAS 39 [member] |  |  |  |  |  |
Disclosure of financial assets [Line Items] |  |  |  |  |  |
Other financial assets |  |  |  |  | 70,241 |
IFRS 9 [member] |  |  |  |  |  |
Disclosure of financial assets [Line Items] |  |  |  |  |  |
Financial assets at amortized cost |  |  |  |  | 70,241 |
IFRS 9 [member] | Increase (decrease) due to changes in accounting policy required by IFRSs [member] |  |  |  |  |  |
Disclosure of financial assets [Line Items] |  |  |  |  |  |
Retained earnings |  |  |  |  | 29,077 |
Other equity interest |  |  |  |  | 42,165 |
Financial assets at fair value through other comprehensive income, category [member] | Increase (decrease) due to changes in accounting policy required by IFRSs [member] |  |  |  |  |  |
Disclosure of financial assets [Line Items] |  |  |  |  |  |
Retained earnings |  |  |  |  | 28,584 |
Other equity interest |  |  |  |  | 42,165 |
Deferred tax liabilities |  |  |  |  | 8,636 |
Financial assets at fair value through other comprehensive income, category [member] | IAS 39 [member] |  |  |  |  |  |
Disclosure of financial assets [Line Items] |  |  |  |  |  |
Available-for-sale financial assets |  |  |  |  | 9,950 |
Financial assets at fair value through other comprehensive income, category [member] | IFRS 9 [member] |  |  |  |  |  |
Disclosure of financial assets [Line Items] |  |  |  |  |  |
Available-for-sale financial assets |  |  |  |  | 89,335 |
Financial assets at fair value through profit or loss, category [member] | Increase (decrease) due to changes in accounting policy required by IFRSs [member] |  |  |  |  |  |
Disclosure of financial assets [Line Items] |  |  |  |  |  |
Retained earnings |  |  |  |  | 493 |
Financial assets at fair value through profit or loss, category [member] | IAS 39 [member] |  |  |  |  |  |
Disclosure of financial assets [Line Items] |  |  |  |  |  |
Available-for-sale financial assets |  |  |  |  | 10,940 |
Financial assets at fair value through profit or loss, category [member] | IFRS 9 [member] |  |  |  |  |  |
Disclosure of financial assets [Line Items] |  |  |  |  |  |
Available-for-sale financial assets |  |  |  |  | $ 11,433 |

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