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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, 2020
TWD ($)
Dec. 31, 2020
USD ($)
Dec. 31, 2019
TWD ($)
Dec. 31, 2018
TWD ($)
Gain (loss) on disposal of financial assets, net        
Investments in debt instruments at FVTOCI $ 1,439.4 $ 51.3 $ 537.8 $ (989.1)
Loss on disposal of subsidiaries     (4.6)  
Gain (loss) on financial instruments at FVTPL, net        
Mandatorily measured at FVTPL 8,244.5   (2,360.7) (2,293.9)
Gain (loss) arising from fair value hedges, net     13.1 (2.3)
The reversal (accrual) of expected credit loss of financial assets        
Investments in debt instruments at FVTOCI 0.9   (6.9) 2.0
Financial assets at amortized cost (4.6)   5.2 0.4
Other gains (losses), net 426.2   665.1 (127.9)
Other gains and losses $ 10,106.4 $ 360.0 $ (1,151.0) $ (3,410.8)