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Financial Instruments - Foreign Currency Risk Sensitivity Analysis (Detail) - TWD ($) $ in Millions | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
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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 | $ 31 | $ 54 | $ 21
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 | (65) | (48) | (62)
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 | (2) | 5 | 5
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 | 0 | 1 | 2
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 | 12
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 | 6 | 3 | 1
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 | 8 | 33
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 | $ 7 | $ 5 | $ 15

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