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Financial Instruments at Fair Value Through Profit or Loss - Outstanding Forward Exchange Contracts Not Designated for Hedge (Detail) - Forward exchange contracts [Member]
€ in Millions, $ in Millions, $ in Millions
 12 Months Ended
Dec. 31, 2020
TWD ($)
Dec. 31, 2019
TWD ($)
Dec. 31, 2020
USD ($)
Dec. 31, 2020
EUR (€)
Dec. 31, 2019
USD ($)
Dec. 31, 2019
EUR (€)
EUR [Member]            
Disclosure of detailed information about financial instruments [Line Items]            
Currency NT$/EUR NT$/EUR        
Maturity Period 2021.03 2020.03        
Contract Amount $ 50 $ 51   € 2   € 2
USD [Member]            
Disclosure of detailed information about financial instruments [Line Items]            
Currency   NT$/US$        
Maturity Period   2020.01        
Contract Amount   $ 26     $ 1  
Taiwan, New Dollars            
Disclosure of detailed information about financial instruments [Line Items]            
Currency US$/NT$          
Maturity Period 2021.02-03          
Contract Amount $ 379   $ 14