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Provisions - Movements in Provisions (Detail) - TWD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Jul. 01, 2020
Disclosure of other provisions [Line Items]        
Beginning balance $ 304 $ 207 $ 180  
Additional/(reversal of) provisions recognized 221 184 192  
Used / forfeited during the year (125) (87) (165)  
Acquired by business combinations (Note 14)       $ 15
Ending balance 415 304 207  
Warranties [Member]        
Disclosure of other provisions [Line Items]        
Beginning balance 173 132 132  
Additional/(reversal of) provisions recognized 131 127 164  
Used / forfeited during the year (122) (86) (164)  
Acquired by business combinations (Note 14)       0
Ending balance 182 173 132  
Onerous Contracts [Member]        
Disclosure of other provisions [Line Items]        
Beginning balance 67 19 0  
Additional/(reversal of) provisions recognized 92 48 19  
Used / forfeited during the year (3) 0 0  
Acquired by business combinations (Note 14)       15
Ending balance 171 67 19  
Employee benefits [Member]        
Disclosure of other provisions [Line Items]        
Beginning balance 59 51 43  
Additional/(reversal of) provisions recognized (2) 9 9  
Used / forfeited during the year 0 (1) (1)  
Acquired by business combinations (Note 14)       0
Ending balance 57 59 51  
Others [Member]        
Disclosure of other provisions [Line Items]        
Beginning balance 5 5 5  
Additional/(reversal of) provisions recognized 0 0 0  
Used / forfeited during the year 0 0 0  
Acquired by business combinations (Note 14)       $ 0
Ending balance $ 5 $ 5 $ 5