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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, 2018
TWD ($)
Dec. 31, 2018
USD ($)
Dec. 31, 2017
TWD ($)
Dec. 31, 2016
TWD ($)
Gain (loss) on disposal of financial assets, net        
Investments in debt instruments at FVTOCI $ (989.1) $ (32.3)    
Available-for-sale financial assets     $ 89.8 $ 33.2
Gain (loss) on disposal of investments accounted for using equity method, net       (260.0)
Gain (loss) from disposal of subsidiaries     17.3 (36.1)
Net gain (loss) on financial instruments at FVTPL        
Held for trading     2,253.7 467.1
Mandatorily measured at FVTPL (2,293.9)      
Designated as at FVTPL     131.0 (37.4)
Gain (loss) arising from fair value hedges, net (2.3)   (30.3) 16.9
Impairment loss on financial assets        
Available-for-sale financial assets     (29.6) (122.2)
The reversal of expected credit loss of financial assets        
Investments in debt instruments at FVTOCI 2.0      
Financial assets at amortized cost 0.4      
Other gains (losses), net (127.9)   385.5 134.4
Other gains and losses $ (3,410.8) $ (111.4) $ 2,817.4 $ 195.9