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Financial Assets and Liabilities at Fair Value through Profit or Loss - Summary of Financial Assets and Liabilities at Fair Value through Profit or Loss (Detail) - TWD ($)
$ in Millions
Dec. 31, 2019
Dec. 31, 2018
Disclosure of detailed information about financial instruments [line items]    
Financial assets, Mandatorily measured at FVTPL $ 326.8 $ 3,504.6
Forward exchange contracts [member]    
Disclosure of detailed information about financial instruments [line items]    
Financial liabilities, Held for trading 982.3 40.8
Financial assets, Mandatorily measured at FVTPL 162.1 85.3
Convertible Bonds [Member]    
Disclosure of detailed information about financial instruments [line items]    
Financial assets, Mandatorily measured at FVTPL 123.8  
Agency bonds/Agency mortgage-backed securities [member]    
Disclosure of detailed information about financial instruments [line items]    
Financial assets, Mandatorily measured at FVTPL $ 40.9 $ 3,419.3