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Hedging Financial Instruments - Summary of Information Related to Hedges of Interest Rate Risk (Detail)
$ in Millions, $ in Millions
 12 Months Ended
Dec. 31, 2020
TWD ($)
Dec. 31, 2020
USD ($)
Dec. 31, 2019
TWD ($)
Dec. 31, 2018
TWD ($)
Dec. 31, 2020
USD ($)
Dec. 31, 2019
USD ($)
Disclosure of detailed information about financial instruments [line items]            
Increase (Decrease) in Value Used for Calculating Hedge Ineffectiveness $ 24.1 $ 0.9 $ (109.6) $ 41.0    
Fair value hedges [member]            
Disclosure of detailed information about financial instruments [line items]            
Increase (Decrease) in Value Used for Calculating Hedge Ineffectiveness     $ 13.1 (2.3)    
US treasury bonds interest rate futures contracts [member]            
Disclosure of detailed information about financial instruments [line items]            
Contract Amount         $ 88.7 $ 122.2
Maturity March 2021 March 2021 March 2020      
US treasury bonds interest rate futures contracts [member] | Fair value hedges [member]            
Disclosure of detailed information about financial instruments [line items]            
Increase (Decrease) in Value Used for Calculating Hedge Ineffectiveness $ (353.6)   $ (164.7) 11.5    
FVTOCI [member]            
Disclosure of detailed information about financial instruments [line items]            
Asset Carrying Amount 6,198.7   7,364.7      
Asset Accumulated Amount of Fair Value Hedge Adjustments 1.1   (22.4)      
FVTOCI [member] | Fair value hedges [member]            
Disclosure of detailed information about financial instruments [line items]            
Increase (Decrease) in Value Used for Calculating Hedge Ineffectiveness $ 353.6   $ 177.8 $ (13.8)