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Other Gains and Losses - Summary of Other Gains and Losses, Net (Detail)
$ in Millions, $ in Millions
 12 Months Ended
Dec. 31, 2019
TWD ($)
Dec. 31, 2019
USD ($)
Dec. 31, 2018
TWD ($)
Dec. 31, 2017
TWD ($)
Gain (loss) on disposal of financial assets, net        
Investments in debt instruments at FVTOCI $ 537.8 $ 18.0 $ (989.1)  
Available-for-sale financial assets       $ 89.8
Gain (loss) from disposal of subsidiaries (4.6)     17.3
Net gain (loss) on financial instruments at FVTPL        
Mandatorily measured at FVTPL (2,360.7)   (2,293.9)  
Held for trading       2,253.7
Designated as at FVTPL       131.0
Gain (loss) arising from fair value hedges, net 13.1   (2.3) (30.3)
Impairment loss on financial assets        
Available-for-sale financial assets       (29.6)
The reversal (accrual) of expected credit loss of financial assets        
Investments in debt instruments at FVTOCI (6.9)   2.0  
Financial assets at amortized cost 5.2   0.4  
Other gains (losses), net 665.1   (127.9) 385.5
Other gains and losses $ (1,151.0) $ (38.5) $ (3,410.8) $ 2,817.4