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Trade Notes and Accounts Receivable, Net - Movements of Allowance for Doubtful Accounts (Detail) - TWD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Disclosure Of Financial Assets [Line Items]      
Beginning balance $ 2,117    
Add: Provision for credit loss or doubtful accounts 805 $ 578 $ 943
Less: Amounts written off (320)    
Ending balance 2,602 2,117  
Trade notes and accounts receivable [Member]      
Disclosure Of Financial Assets [Line Items]      
Beginning balance 2,117 1,773 1,334
Add: Provision for credit loss or doubtful accounts   578 943
Less: Amounts written off   (234) (504)
Ending balance   2,117 1,773
Trade notes and accounts receivable [Member] | Individually assessed for impairment [Member]      
Disclosure Of Financial Assets [Line Items]      
Beginning balance 1,325 805 364
Add: Provision for credit loss or doubtful accounts   535 715
Less: Amounts written off   (15) (274)
Ending balance   1,325 805
Trade notes and accounts receivable [Member] | Collectively assessed for impairment [Member]      
Disclosure Of Financial Assets [Line Items]      
Beginning balance $ 792 968 970
Add: Provision for credit loss or doubtful accounts   43 228
Less: Amounts written off   (219) (230)
Ending balance   $ 792 $ 968