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Net Income - Other Income and Expenses (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]        
Gain (loss) on disposal of property, plant and equipment $ 1,428 $ 51 $ (38) $ 142
Impairment loss on property, plant and equipment 0 0 (93) 0
Gain on disposal of investment properties 151 5 0 0
Reversal of impairment loss on investment properties 27 1 57 19
Loss on disposal of intangible assets (2) 0 0 0
Impairment loss on intangible assets (9) 0 (9) (51)
Impairment loss on other assets 0 0 (44) 0
Other income and expenses $ 1,595 $ 57 $ (127) $ 110