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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, 2019 TWD ($) | Dec. 31, 2019 USD ($) | Dec. 31, 2018 TWD ($) | Dec. 31, 2017 TWD ($)
Disclosure of fair value measurement of assets [line items] |  |  |  |
Beginning Balance | $ 534,061,927 |  |  |
Net increase in trade receivables | 995,839 | $ 33,295 | $ (9,313,539) | $ (4,066,374)
Ending Balance | 557,223,706 | 18,630,014 | 534,061,927 |
Available-for-sale [member] | Level 3 of fair value hierarchy [member] |  |  |  |
Disclosure of fair value measurement of assets [line items] |  |  |  |
Beginning Balance | 2,072,446 | 69,290 | 1,988,549 |
Recognized in profit or loss | 3,431 | 115 | (2,313) |
Recognized in other comprehensive income (included in unrealized losses on financial assets at FVTOCI) | (218,173) | (7,295) | (287,248) |
Recognized in other comprehensive income (effects of foreign currency exchange) | (20,063) | (670) |  |
Net increase in trade receivables | 3,171,205 | 106,025 |  |
Purchases | 951,513 | 31,812 | 582,436 |
Trade receivables factoring (purchases) | (1,141,515) | (38,165) |  |
Disposals | (12,665) | (424) | (208,978) |
Ending Balance | 4,806,179 | 160,687 | 2,072,446 | 1,988,549
Available-for-sale [member] | Level 3 of fair value hierarchy [member] | FVTPL equity instrument [member] |  |  |  |
Disclosure of fair value measurement of assets [line items] |  |  |  |
Beginning Balance | 475,123 | 15,885 |  |
Recognized in profit or loss | 3,431 | 115 | (2,313) |
Recognized in other comprehensive income (effects of foreign currency exchange) | (14,368) | (480) |  |
Purchases | 541,528 | 18,105 | 477,436 |
Ending Balance | 1,005,714 | 33,624 | 475,123 |
Available-for-sale [member] | Level 3 of fair value hierarchy [member] | FVOCI equity instruments [member] |  |  |  |
Disclosure of fair value measurement of assets [line items] |  |  |  |
Beginning Balance | 580,399 | 19,405 | 908,549 |
Recognized in other comprehensive income (included in unrealized losses on financial assets at FVTOCI) | (216,121) | (7,226) | (224,172) |
Recognized in other comprehensive income (effects of foreign currency exchange) | (5,695) | (190) |  |
Purchases | 409,985 | 13,707 | 105,000 |
Disposals | (12,665) | (424) | (208,978) |
Ending Balance | 755,903 | 25,272 | 580,399 | 908,549
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 | 1,016,924 | 34,000 | 1,080,000 |
Recognized in other comprehensive income (included in unrealized losses on financial assets at FVTOCI) | (2,052) | (69) | (63,076) |
Net increase in trade receivables | 3,171,205 | 106,025 |  |
Trade receivables factoring (purchases) | (1,141,515) | (38,165) |  |
Ending Balance | $ 3,044,562 | $ 101,791 | $ 1,016,924 | $ 1,080,000

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