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Financial Instruments - Liquidity Analysis for Derivative Financial Instruments (Detail) - TWD ($) $ in Millions | Dec. 31, 2018 | Dec. 31, 2017
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Disclosure of maturity analysis for derivative financial liabilities [Line Items] |  |
Undiscounted gross inflows (outflows) on derivatives | $ 0 | $ (2)
Forward exchange contracts inflows [Member] |  |
Disclosure of maturity analysis for derivative financial liabilities [Line Items] |  |
Undiscounted gross inflows (outflows) on derivatives | 426 | 334
Forward exchange contracts outflows [Member] |  |
Disclosure of maturity analysis for derivative financial liabilities [Line Items] |  |
Undiscounted gross inflows (outflows) on derivatives | 426 | 336
Less Than 1 Month [Member] |  |
Disclosure of maturity analysis for derivative financial liabilities [Line Items] |  |
Undiscounted gross inflows (outflows) on derivatives | 0 | (1)
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 | 62 | 125
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 | 62 | 126
1-3 Months [Member] |  |
Disclosure of maturity analysis for derivative financial liabilities [Line Items] |  |
Undiscounted gross inflows (outflows) on derivatives | 0 | (1)
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 | 238 | 173
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 | 238 | 174
3 Months to 1 Year [Member] |  |
Disclosure of maturity analysis for derivative financial liabilities [Line Items] |  |
Undiscounted gross inflows (outflows) on derivatives | 0 | 0
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 | 126 | 36
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 | 126 | 36
1-5 Years [Member] |  |
Disclosure of maturity analysis for derivative financial liabilities [Line Items] |  |
Undiscounted gross inflows (outflows) on derivatives | 0 | 0
1-5 Years [Member] | Forward exchange contracts inflows [Member] |  |
Disclosure of maturity analysis for derivative financial liabilities [Line Items] |  |
Undiscounted gross inflows (outflows) on derivatives | 0 | 0
1-5 Years [Member] | Forward exchange contracts outflows [Member] |  |
Disclosure of maturity analysis for derivative financial liabilities [Line Items] |  |
Undiscounted gross inflows (outflows) on derivatives | $ 0 | $ 0

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