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Income Tax - Additional Information (Detail) $ in Thousands, $ in Thousands | 1 Months Ended | 12 Months Ended |
--- | --- | --- | ---
Feb. 28, 2018 | Dec. 31, 2017 TWD ($) | Dec. 31, 2017 USD ($) | Dec. 31, 2016 TWD ($) | Dec. 31, 2015 | Dec. 31, 2017 USD ($)
Disclosure of income taxes [line items] |  |  |  |  |  |
Applied tax rate | 20.00% | 17.00% | 17.00% | 17.00% | 17.00% |
Income tax on unappropriated earnings | 5.00% | 10.00% | 10.00% |  |  |
Increase in deferred tax assets |  | $ 201,965 | $ 6,814 |  |  |
Increase in deferred tax liabilities |  | 788,556 |  |  |  | $ 26,604
Taxable temporary differences associated with the investments in subsidiaries for which no deferred tax liabilities recognized |  | 16,401,422 |  | $ 14,417,873 |  | 553,354
Balance of Imputation Credit Account |  | $ 4,003,283 |  | $ 3,328,374 |  | $ 135,064
Ratio of distribution of earnings |  |  |  | 10.01% |  |
Subsidiaries [member] | China [member] |  |  |  |  |  |
Disclosure of income taxes [line items] |  |  |  |  |  |
Applied tax rate |  | 25.00% | 25.00% | 25.00% | 25.00% |
Reduction tax rate |  | 15.00% | 15.00% |  |  |

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