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Financial Instruments - Foreign Currency Risk Sensitivity Analysis (Detail) - TWD ($) $ in Millions | Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
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Profit and loss [Member] | Financial instruments [Member] | USD [Member] |  |  |
Disclosure of detailed information about financial instruments [Line Items] |  |  |
Sensitivity analysis due to a 5% increase or decrease in foreign currency exchange | $ 97 | $ 83 | $ (55)
Profit and loss [Member] | Financial instruments [Member] | EUR [Member] |  |  |
Disclosure of detailed information about financial instruments [Line Items] |  |  |
Sensitivity analysis due to a 5% increase or decrease in foreign currency exchange | (47) | (10) | (59)
Profit and loss [Member] | Financial instruments [Member] | SGD [Member] |  |  |
Disclosure of detailed information about financial instruments [Line Items] |  |  |
Sensitivity analysis due to a 5% increase or decrease in foreign currency exchange | (44) | (52) | 4
Profit and loss [Member] | Financial instruments [Member] | RMB [Member] |  |  |
Disclosure of detailed information about financial instruments [Line Items] |  |  |
Sensitivity analysis due to a 5% increase or decrease in foreign currency exchange | 1 | 0 | 0
Profit and loss [Member] | Financial instruments [Member] | JPY [Member] |  |  |
Disclosure of detailed information about financial instruments [Line Items] |  |  |
Sensitivity analysis due to a 5% increase or decrease in foreign currency exchange | 1 | 0 | 0
Profit and loss [Member] | Financial instruments [Member] | HKD [Member] |  |  |
Disclosure of detailed information about financial instruments [Line Items] |  |  |
Sensitivity analysis due to a 5% increase or decrease in foreign currency exchange | 3 | (1) | 0
Profit and loss [Member] | Derivatives [Member] | USD [Member] |  |  |
Disclosure of detailed information about financial instruments [Line Items] |  |  |
Sensitivity analysis due to a 5% increase or decrease in foreign currency exchange | (19) | 1 | 3
Profit and loss [Member] | Derivatives [Member] | EUR [Member] |  |  |
Disclosure of detailed information about financial instruments [Line Items] |  |  |
Sensitivity analysis due to a 5% increase or decrease in foreign currency exchange | 3 | 3 | 10
Equity [member] | Derivatives [Member] | EUR [Member] |  |  |
Disclosure of detailed information about financial instruments [Line Items] |  |  |
Sensitivity analysis due to a 5% increase or decrease in foreign currency exchange | $ 10 | $ 4 | $ 9

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