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Income Tax - Information About Unused Loss Carryforwards (Detail) $ in Millions, $ in Millions | Dec. 31, 2017 TWD ($) | Dec. 31, 2017 USD ($) | Dec. 31, 2016 TWD ($)
--- | --- | --- | ---
Disclosure of temporary difference, unused tax losses and unused tax credits [Line Items] |  |  |
Remaining creditable amount | $ 2,730 | $ 92 | $ 2,322
Unused tax losses [Member] |  |  |
Disclosure of temporary difference, unused tax losses and unused tax credits [Line Items] |  |  |
Remaining creditable amount | 387 |  |
Unused tax losses [Member] | 2018 [Member] |  |  |
Disclosure of temporary difference, unused tax losses and unused tax credits [Line Items] |  |  |
Remaining creditable amount | 126 |  |
Unused tax losses [Member] | 2019 [Member] |  |  |
Disclosure of temporary difference, unused tax losses and unused tax credits [Line Items] |  |  |
Remaining creditable amount | 138 |  |
Unused tax losses [Member] | 2020 [Member] |  |  |
Disclosure of temporary difference, unused tax losses and unused tax credits [Line Items] |  |  |
Remaining creditable amount | 45 |  |
Unused tax losses [Member] | 2021 [Member] |  |  |
Disclosure of temporary difference, unused tax losses and unused tax credits [Line Items] |  |  |
Remaining creditable amount | 20 |  |
Unused tax losses [Member] | 2022 [Member] |  |  |
Disclosure of temporary difference, unused tax losses and unused tax credits [Line Items] |  |  |
Remaining creditable amount | 12 |  |
Unused tax losses [Member] | 2023 [Member] |  |  |
Disclosure of temporary difference, unused tax losses and unused tax credits [Line Items] |  |  |
Remaining creditable amount | 1 |  |
Unused tax losses [Member] | 2024 [Member] |  |  |
Disclosure of temporary difference, unused tax losses and unused tax credits [Line Items] |  |  |
Remaining creditable amount | 4 |  |
Unused tax losses [Member] | 2025 [Member] |  |  |
Disclosure of temporary difference, unused tax losses and unused tax credits [Line Items] |  |  |
Remaining creditable amount | 28 |  |
Unused tax losses [Member] | 2026 [Member] |  |  |
Disclosure of temporary difference, unused tax losses and unused tax credits [Line Items] |  |  |
Remaining creditable amount | 10 |  |
Unused tax losses [Member] | 2027 [Member] |  |  |
Disclosure of temporary difference, unused tax losses and unused tax credits [Line Items] |  |  |
Remaining creditable amount | $ 3 |  |

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