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Provisions - Movements in Provisions (Detail) - TWD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Jan. 01, 2018
Disclosure of other provisions [Line Items]        
Beginning balance $ 267 $ 185 $ 248  
Effect of retrospective application of IFRS 15       $ (87)
Additional provisions recognized 192 155 121  
Used / forfeited during the year (165) (73) (184)  
Ending balance 207 267 185  
Beginning balance as adjusted       180
Warranties [Member]        
Disclosure of other provisions [Line Items]        
Beginning balance 132 111 213  
Effect of retrospective application of IFRS 15       0
Additional provisions recognized 164 79 81  
Used / forfeited during the year (164) (58) (183)  
Ending balance 132 132 111  
Beginning balance as adjusted       132
Employee benefits [Member]        
Disclosure of other provisions [Line Items]        
Beginning balance 43 38 30  
Effect of retrospective application of IFRS 15       0
Additional provisions recognized 9 7 9  
Used / forfeited during the year (1) (2) (1)  
Ending balance 51 43 38  
Beginning balance as adjusted       43
Trade-in rights [Member]        
Disclosure of other provisions [Line Items]        
Beginning balance 87 31 0  
Effect of retrospective application of IFRS 15       (87)
Additional provisions recognized 0 69 31  
Used / forfeited during the year 0 (13) 0  
Ending balance 0 87 31  
Beginning balance as adjusted       0
Others [Member]        
Disclosure of other provisions [Line Items]        
Beginning balance 5 5 5  
Effect of retrospective application of IFRS 15       0
Additional provisions recognized 19 0 0  
Used / forfeited during the year 0 0 0  
Ending balance $ 24 $ 5 $ 5  
Beginning balance as adjusted       $ 5