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Effects on Initial application of IFRS 9 - Summary of Carrying Amount of Financial Assets Transferred (Detail) $ in Thousands, $ in Thousands | Dec. 31, 2020 TWD ($) | Dec. 31, 2020 USD ($) | Dec. 31, 2019 TWD ($) | Jan. 01, 2018 TWD ($)
--- | --- | --- | --- | ---
Disclosure of financial assets [Line Items] |  |  |  |
Measured at fair value through profit or loss | $ 63,488 |  | $ 11,038 |
Measured at fair value through other comprehensive income | 262,007 |  | 121,808 |
Measured at amortized cost | 254,801 |  | 237,420 |
Financial assets | 10,131,324 |  | 9,642,833 |
Retained earnings | $ 5,401,569 | $ 192,364 | $ 4,651,215 |
Fair value adjustment |  |  |  |
Disclosure of financial assets [Line Items] |  |  |  |
Measured at fair value through profit or loss |  |  |  | $ 493
Measured at fair value through other comprehensive income |  |  |  | 50,801
Financial assets |  |  |  | 51,294
Impairment loss adjustment [member] |  |  |  |
Disclosure of financial assets [Line Items] |  |  |  |
Measured at fair value through other comprehensive income |  |  |  | 28,584
Financial assets |  |  |  | 28,584
IAS 39 [member] |  |  |  |
Disclosure of financial assets [Line Items] |  |  |  |
Measured at cost |  |  |  | 20,890
Other financial assets |  |  |  | 70,241
Financial assets |  |  |  | 91,131
IFRS 9 [member] |  |  |  |
Disclosure of financial assets [Line Items] |  |  |  |
Measured at fair value through profit or loss |  |  |  | 11,433
Measured at fair value through other comprehensive income |  |  |  | 89,335
Measured at amortized cost |  |  |  | 70,241
Financial assets |  |  |  | 171,009
Transferred into and measured at fair value through profit or loss [member] |  |  |  |
Disclosure of financial assets [Line Items] |  |  |  |
Measured at cost |  |  |  | (10,940)
Measured at fair value through profit or loss |  |  |  | 10,940
Transferred into and measured at fair value through other comprehensive income [member] |  |  |  |
Disclosure of financial assets [Line Items] |  |  |  |
Measured at cost |  |  |  | (9,950)
Measured at fair value through other comprehensive income |  |  |  | 9,950
Transfer into and measured at amortized cost [member] |  |  |  |
Disclosure of financial assets [Line Items] |  |  |  |
Other financial assets |  |  |  | (70,241)
Measured at amortized cost |  |  |  | 70,241
Increase (decrease) due to changes in accounting policy required by IFRSs [member] |  |  |  |
Disclosure of financial assets [Line Items] |  |  |  |
Retained earnings |  |  |  | (1,940)
Increase (decrease) due to changes in accounting policy required by IFRSs [member] | Fair value adjustment |  |  |  |
Disclosure of financial assets [Line Items] |  |  |  |
Retained earnings |  |  |  | 493
Other equity interest |  |  |  | 79,385
Increase (decrease) due to changes in accounting policy required by IFRSs [member] | Impairment loss adjustment [member] |  |  |  |
Disclosure of financial assets [Line Items] |  |  |  |
Retained earnings |  |  |  | 28,584
Other equity interest |  |  |  | (28,584)
Increase (decrease) due to changes in accounting policy required by IFRSs [member] | Income tax adjustment [member] |  |  |  |
Disclosure of financial assets [Line Items] |  |  |  |
Other equity interest |  |  |  | (8,636)
Increase (decrease) due to changes in accounting policy required by IFRSs [member] | IFRS 9 [member] |  |  |  |
Disclosure of financial assets [Line Items] |  |  |  |
Retained earnings |  |  |  | 29,077
Other equity interest |  |  |  | $ 42,165

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