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Investments Accounted for Using the Equity Method - Additional Information (Detail) $ in Thousands, $ in Thousands |  | 1 Months Ended | 12 Months Ended
--- | --- | --- | ---
Feb. 28, 2019 USD ($) | Feb. 28, 2019 TWD ($) | Oct. 31, 2014 USD ($) | Oct. 31, 2014 TWD ($) | Dec. 31, 2018 USD ($) | Dec. 31, 2018 TWD ($) | Dec. 31, 2017 TWD ($) | Dec. 31, 2016 TWD ($)
Disclosure of Investments Accounted for Using Equity Method [line Items] |  |  |  |  |  |  |  |
Impairment losses on financial assets |  |  |  |  | $ 17,021 | $ 521,010 | $ 50,206 | $ 91,886
Siliconware Precision Industries Co., Ltd. [member] |  |  |  |  |  |  |  |
Disclosure of Investments Accounted for Using Equity Method [line Items] |  |  |  |  |  |  |  |
Gain recognized from remeasuring fair value of equity interest before business combination |  |  |  |  | $ 242,451 | $ 7,421,408 |  |
Percentage acquisition of shareholding |  |  |  |  | 100.00% | 100.00% |  |
Consideration paid in cash for shares |  |  | $ 170,434 | $ 5,216,985 |  |  |  |
Percentage of ownership interest in associate |  |  |  |  |  |  | 33.29% |
ChipMos Technologies Inc [member] |  |  |  |  |  |  |  |
Disclosure of Investments Accounted for Using Equity Method [line Items] |  |  |  |  |  |  |  |
Ordinary shares acquired |  |  |  |  |  | $ 19,159 |  |
Consideration paid in cash for shares |  |  |  |  | $ 14,752 | $ 451,563 |  |
Percentage of ownership interest in associate |  |  |  |  | 20.47% | 20.47% |  |
ChipMos Technologies Inc [member] | Bottom of range [member] |  |  |  |  |  |  |  |
Disclosure of Investments Accounted for Using Equity Method [line Items] |  |  |  |  |  |  |  |
Percentage of ownership interest in associate |  |  |  |  | 18.23% | 18.23% |  |
ChipMos Technologies Inc [member] | Top of range [member] |  |  |  |  |  |  |  |
Disclosure of Investments Accounted for Using Equity Method [line Items] |  |  |  |  |  |  |  |
Percentage of ownership interest in associate |  |  |  |  | 20.47% | 20.47% |  |
Deca Technologies Inc. [member] |  |  |  |  |  |  |  |
Disclosure of Investments Accounted for Using Equity Method [line Items] |  |  |  |  |  |  |  |
Percentage of ownership interest in associate |  |  |  |  | 22.04% | 22.04% | 22.04% |
Assets discount rate |  |  |  |  | 14.10% | 14.10% |  |
ASE Embedded Electronics Inc. [member] |  |  |  |  |  |  |  |
Disclosure of Investments Accounted for Using Equity Method [line Items] |  |  |  |  |  |  |  |
Percentage of ownership interest in joint venture |  |  |  |  | 51.00% | 51.00% | 51.00% |
ASE Embedded Electronics Inc. [member] | Major ordinary share transactions [member] |  |  |  |  |  |  |  |
Disclosure of Investments Accounted for Using Equity Method [line Items] |  |  |  |  |  |  |  |
Consideration paid in cash for shares | $ 49,004 | $ 1,500,000 |  |  |  |  |  |
ASE Embedded Electronics Inc. [member] | Major ordinary share transactions [member] | Top of range [member] |  |  |  |  |  |  |  |
Disclosure of Investments Accounted for Using Equity Method [line Items] |  |  |  |  |  |  |  |
Ordinary shares acquired |  | $ 150,000 |  |  |  |  |  |

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