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Financial Instruments - Summary of Financial Assets Measured at Level 3 Fair Value (Detail) $ in Thousands, $ in Thousands | 12 Months Ended
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Dec. 31, 2018 USD ($) | Dec. 31, 2018 TWD ($) | Dec. 31, 2017 TWD ($) | Dec. 31, 2016 TWD ($)
Disclosure of fair value measurement of assets [line items] |  |  |  |
Beginning Balance |  | $ 363,922,272 |  |
Ending Balance | $ 17,447,301 | 534,061,927 | $ 363,922,272 |
Available-for-sale [member] | Level 3 of fair value hierarchy [member] |  |  |  |
Disclosure of fair value measurement of assets [line items] |  |  |  |
Beginning Balance | 64,964 | 1,988,549 |  |
Recognized in profit or loss | (76) | (2,313) |  |
Recognized in other comprehensive income (included in unrealized losses on financial assets at FVTOCI) | (9,384) | (287,248) |  |
Purchases | 19,027 | 582,436 |  |
Disposals | (6,827) | (208,978) |  |
Ending Balance | 67,704 | 2,072,446 | 1,988,549 |
Available-for-sale [member] | Level 3 of fair value hierarchy [member] | Equity Instruments Fair Value Other Comprehensive Income [member] |  |  |  |
Disclosure of fair value measurement of assets [line items] |  |  |  |
Beginning Balance | 29,681 | 908,549 | 904,790 | $ 741,089
Recognized in profit or loss |  |  | 28 | (100,734)
Recognized in other comprehensive income (included in unrealized losses on financial assets at FVTOCI) | (7,323) | (224,172) | 17,284 | (202,565)
Purchases | 3,430 | 105,000 | 2,649 | 495,928
Disposals | (6,827) | (208,978) | 16,202 | 28,928
Ending Balance | 18,961 | 580,399 | 908,549 | $ 904,790
Available-for-sale [member] | Level 3 of fair value hierarchy [member] | FVTPL equity instrument [member] |  |  |  |
Disclosure of fair value measurement of assets [line items] |  |  |  |
Recognized in profit or loss | (76) | (2,313) |  |
Purchases | 15,597 | 477,436 |  |
Ending Balance | 15,521 | 475,123 |  |
Available-for-sale [member] | Level 3 of fair value hierarchy [member] | FVOCI Debt Instruments [member] |  |  |  |
Disclosure of fair value measurement of assets [line items] |  |  |  |
Beginning Balance | 35,283 | 1,080,000 |  |
Recognized in other comprehensive income (included in unrealized losses on financial assets at FVTOCI) | (2,061) | (63,076) |  |
Ending Balance | $ 33,222 | $ 1,016,924 | $ 1,080,000 |

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