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Financial Instruments - Liquidity Analysis for Derivative Financial Instruments (Detail) - TWD ($)
$ in Millions
Dec. 31, 2020
Dec. 31, 2019
Disclosure of maturity analysis for derivative financial liabilities [Line Items]    
Undiscounted gross inflows (outflows) on derivatives $ 4 $ 0
Forward exchange contracts inflows [Member]    
Disclosure of maturity analysis for derivative financial liabilities [Line Items]    
Undiscounted gross inflows on derivatives 635 161
Forward exchange contracts outflows [Member]    
Disclosure of maturity analysis for derivative financial liabilities [Line Items]    
Undiscounted gross outflows on derivatives 631 161
Less Than 1 Month [Member]    
Disclosure of maturity analysis for derivative financial liabilities [Line Items]    
Undiscounted gross inflows (outflows) on derivatives 0 0
Less Than 1 Month [Member] | Forward exchange contracts inflows [Member]    
Disclosure of maturity analysis for derivative financial liabilities [Line Items]    
Undiscounted gross inflows on derivatives 0 26
Less Than 1 Month [Member] | Forward exchange contracts outflows [Member]    
Disclosure of maturity analysis for derivative financial liabilities [Line Items]    
Undiscounted gross outflows on derivatives 0 26
1-3 Months [Member]    
Disclosure of maturity analysis for derivative financial liabilities [Line Items]    
Undiscounted gross inflows (outflows) on derivatives 4 0
1-3 Months [Member] | Forward exchange contracts inflows [Member]    
Disclosure of maturity analysis for derivative financial liabilities [Line Items]    
Undiscounted gross inflows on derivatives 635 135
1-3 Months [Member] | Forward exchange contracts outflows [Member]    
Disclosure of maturity analysis for derivative financial liabilities [Line Items]    
Undiscounted gross outflows on derivatives 631 135
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 on derivatives 0 0
3 Months to 1 Year [Member] | Forward exchange contracts outflows [Member]    
Disclosure of maturity analysis for derivative financial liabilities [Line Items]    
Undiscounted gross outflows on derivatives 0 0
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 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 outflows on derivatives $ 0 $ 0