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Financial Instruments - Additional Information (Detail) - TWD ($) $ in Millions | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
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Foreign currency risk [Member] |  |  |
Disclosure of detailed information about financial instruments [Line Items] |  |  |
Sensitivity rate | 5.00% | 5.00% | 5.00%
Interest rate risk [Member] |  |  |
Disclosure of detailed information about financial instruments [Line Items] |  |  |
Sensitivity rate | 0.25% | 0.25% | 0.25%
Increase decrease in pre-tax income due to sensitivity analysis | $ 13 | $ 12 | $ 12
Equity price risk [Member] |  |  |
Disclosure of detailed information about financial instruments [Line Items] |  |  |
Sensitivity rate | 5.00% | 5.00% | 5.00%
Increase decrease in pre-tax income due to sensitivity analysis | $ 156 | $ 126 | $ 162

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