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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, 2019
Dec. 31, 2018
Dec. 31, 2017
Disclosure Of Financial Assets [Line Items]      
Beginning balance $ 2,602    
Add: Provision for (reversal of) credit loss (54) $ 805 $ 578
Ending balance 2,360 2,602  
Trade notes and accounts receivable [Member]      
Disclosure Of Financial Assets [Line Items]      
Beginning balance 2,602 2,117 1,773
Add: Provision for (reversal of) credit loss (54) 805 578
Less: Amounts written off (188) (320) (234)
Ending balance $ 2,360 2,602 2,117
Trade notes and accounts receivable [Member] | Individually assessed for impairment [Member]      
Disclosure Of Financial Assets [Line Items]      
Beginning balance   1,325 805
Add: Provision for (reversal of) credit loss     535
Less: Amounts written off     (15)
Ending balance     1,325
Trade notes and accounts receivable [Member] | Collectively assessed for impairment [Member]      
Disclosure Of Financial Assets [Line Items]      
Beginning balance   $ 792 968
Add: Provision for (reversal of) credit loss     43
Less: Amounts written off     (219)
Ending balance     $ 792