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Net Income - Other Gains and Losses (Detail)
$ in Millions, $ in Millions
 12 Months Ended
Dec. 31, 2020
TWD ($)
Dec. 31, 2020
USD ($)
Dec. 31, 2019
TWD ($)
Dec. 31, 2018
TWD ($)
Analysis Of Income And Expense [Abstract]        
Foreign currency exchange gain or loss, net $ (47)   $ 16 $ 37
Gain or loss on disposal of investments accounted for using equity method 10 $ 0 151 0
Gain (loss) on disposal of financial instruments (2) 0 4 6
Valuation gain or loss on financial assets and liabilities at fair value through profit or loss, net (99) (4) (38) (21)
Others (21)   (49) (68)
Other gains and losses $ (159) $ (6) $ 84 $ (46)