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Trade Notes and Accounts Receivable, Net - Movements of Allowance for Doubtful Accounts, Trade Notes and Accounts Receivable (Detail) - TWD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Disclosure Of Financial Assets [Line Items]      
Beginning balance $ 2,360    
Add: Provision for (reversal of) credit loss 49 $ (54) $ 805
Ending balance 2,154 2,360  
Trade notes and accounts receivable [Member]      
Disclosure Of Financial Assets [Line Items]      
Beginning balance 2,360 2,602  
Add: Provision for (reversal of) credit loss 49 (54)  
Add: Acquired by business combinations (Note 14) 2    
Less: Amounts written off (257) (188)  
Ending balance $ 2,154 $ 2,360 $ 2,602