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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, 2017
TWD ($)
Dec. 31, 2017
USD ($)
Dec. 31, 2016
TWD ($)
Dec. 31, 2015
TWD ($)
Gain on disposal of financial assets, net        
Available-for-sale financial assets $ 89.8 $ 3.0 $ 33.2 $ 22,157.9
Gain (loss) on disposal of investments accounted for using equity method, net     (260.0) 2,492.1
Gain (loss) from disposal of subsidiaries 17.3   (36.1) (138.2)
Other gains 409.9   176.8 189.3
Net gain (loss) on financial instruments at FVTPL        
Held for trading 2,253.7   467.1 (1,769.3)
Designated as at FVTPL 131.0   (37.4)  
Gain (loss) arising from fair value hedges, net (30.3)   16.9 (439.7)
Impairment loss of financial assets        
Available-for-sale financial assets (29.6) (1.0) (122.2) (154.7)
Other losses (24.4)   (42.4) (145.9)
Other gains and losses $ 2,817.4 $ 95.1 $ 195.9 $ 22,191.5