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Financial Instruments - Additional Information (Detail) - TWD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017
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 | $ 15 | $ 19 | $ 13
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 | $ 0 | $ 0 | $ 156
Equity price risk [Member] | Pre-tax profit [Member] |  |  |
Disclosure of detailed information about financial instruments [Line Items] |  |  |
Increase decrease in pretax income of financial assets at FVTPL, FVTOCI | 39 | 26 | 0
Equity price risk [Member] | Pre-tax other comprehensive income [Member] |  |  |
Disclosure of detailed information about financial instruments [Line Items] |  |  |
Increase decrease in pretax income of financial assets at FVTPL, FVTOCI | $ 358 | $ 347 | $ 0

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