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Financial Instruments - Liquidity Analysis for Derivative Financial Instruments (Detail) - TWD ($) $ in Millions | Dec. 31, 2017 | Dec. 31, 2016
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Disclosure of maturity analysis for derivative financial liabilities [Line Items] |  |
Undiscounted gross inflows (outflows) on derivatives | $ (2) | $ (2)
Forward exchange contracts inflows [Member] |  |
Disclosure of maturity analysis for derivative financial liabilities [Line Items] |  |
Undiscounted gross inflows (outflows) on derivatives | 334 | 322
Forward exchange contracts outflows [Member] |  |
Disclosure of maturity analysis for derivative financial liabilities [Line Items] |  |
Undiscounted gross inflows (outflows) on derivatives | 336 | 324
Less Than 1 Month [Member] |  |
Disclosure of maturity analysis for derivative financial liabilities [Line Items] |  |
Undiscounted gross inflows (outflows) on derivatives | (1) | 0
Less Than 1 Month [Member] | Forward exchange contracts inflows [Member] |  |
Disclosure of maturity analysis for derivative financial liabilities [Line Items] |  |
Undiscounted gross inflows (outflows) on derivatives | 125 | 55
Less Than 1 Month [Member] | Forward exchange contracts outflows [Member] |  |
Disclosure of maturity analysis for derivative financial liabilities [Line Items] |  |
Undiscounted gross inflows (outflows) on derivatives | 126 | 55
1-3 Months [Member] |  |
Disclosure of maturity analysis for derivative financial liabilities [Line Items] |  |
Undiscounted gross inflows (outflows) on derivatives | (1) | (2)
1-3 Months [Member] | Forward exchange contracts inflows [Member] |  |
Disclosure of maturity analysis for derivative financial liabilities [Line Items] |  |
Undiscounted gross inflows (outflows) on derivatives | 173 | 267
1-3 Months [Member] | Forward exchange contracts outflows [Member] |  |
Disclosure of maturity analysis for derivative financial liabilities [Line Items] |  |
Undiscounted gross inflows (outflows) on derivatives | 174 | 269
3 Months to 1 Year [Member] | Forward exchange contracts inflows [Member] |  |
Disclosure of maturity analysis for derivative financial liabilities [Line Items] |  |
Undiscounted gross inflows (outflows) on derivatives | 36 | 0
3 Months to 1 Year [Member] | Forward exchange contracts outflows [Member] |  |
Disclosure of maturity analysis for derivative financial liabilities [Line Items] |  |
Undiscounted gross inflows (outflows) on derivatives | $ 36 | $ 0

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- Definition Line items represent concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes of the table. + References No definition available. + Details
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