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Financial Assets and Liabilities at Fair Value through Profit or Loss - Outstanding Forward Exchange Contracts for which Hedge Accounting Treatment is not Applied (Detail)
¥ in Millions, $ in Millions, $ in Millions
 12 Months Ended
Dec. 31, 2020
TWD ($)
Dec. 31, 2019
TWD ($)
Dec. 31, 2020
USD ($)
Dec. 31, 2019
USD ($)
Dec. 31, 2019
JPY (¥)
Sell NT$ | Bottom of range [member]          
Disclosure of detailed information about financial instruments [line items]          
Maturity Date 2021-01 2020-01      
Sell NT$ | Top of range [member]          
Disclosure of detailed information about financial instruments [line items]          
Maturity Date 2021-03 2020-06      
Sell NT$ | Sell [member]          
Disclosure of detailed information about financial instruments [line items]          
Contract Amount $ 144,698.0 $ 108,428.0      
Sell JPY | Bottom of range [member]          
Disclosure of detailed information about financial instruments [line items]          
Maturity Date   2020-01      
Sell JPY | Top of range [member]          
Disclosure of detailed information about financial instruments [line items]          
Maturity Date   2020-02      
Sell JPY | Sell [member]          
Disclosure of detailed information about financial instruments [line items]          
Contract Amount | ¥         ¥ 57,471.6
Sell US$ | Bottom of range [member]          
Disclosure of detailed information about financial instruments [line items]          
Maturity Date 2021-01 2020-01      
Sell US$ | Top of range [member]          
Disclosure of detailed information about financial instruments [line items]          
Maturity Date 2021-03 2020-03      
Sell US$ | Sell [member]          
Disclosure of detailed information about financial instruments [line items]          
Contract Amount     $ 1,176.9 $ 529.2