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Hedging Financial Instruments - Summary of Information Relating to Hedges for Foreign Currency Risk (Detail)
$ in Millions, $ in Millions
 12 Months Ended
Dec. 31, 2020
TWD ($)
Dec. 31, 2020
USD ($)
Dec. 31, 2019
TWD ($)
Dec. 31, 2018
TWD ($)
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
Cash flow hedges [member]        
Disclosure of detailed information about financial instruments [line items]        
Increase (Decrease) in Value Used for Calculating Hedge Ineffectiveness 24.1   (109.6) 41.0
Cash flow hedges [member] | Forecast Transaction Capital Expenditures [member]        
Disclosure of detailed information about financial instruments [line items]        
Increase (Decrease) in Value Used for Calculating Hedge Ineffectiveness (24.1)   109.6 (41.0)
Cash flow hedges [member] | Forward exchange contracts [member]        
Disclosure of detailed information about financial instruments [line items]        
Contract Amount     $ 1,342.4  
Maturity     January 2020  
Balance in Other Equity (Continuing Hedges)     $ (3.8)  
Increase (Decrease) in Value Used for Calculating Hedge Ineffectiveness $ 24.1   $ (109.6) 34.6
Cash flow hedges [member] | Foreign currency deposits [Member]        
Disclosure of detailed information about financial instruments [line items]        
Increase (Decrease) in Value Used for Calculating Hedge Ineffectiveness       $ 6.4