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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, 2017
Dec. 31, 2016
Dec. 31, 2015
Disclosure of financial assets [Line Items]      
Add: Provision for doubtful accounts $ 578 $ 943 $ 480
Trade notes and accounts receivable [Member]      
Disclosure of financial assets [Line Items]      
Beginning balance 1,773 1,334 1,049
Add: Provision for doubtful accounts 578 943 480
Deduct: Amounts written off (234) (504) (195)
Ending balance 2,117 1,773 1,334
Trade notes and accounts receivable [Member] | Individually assessed for impairment [Member]      
Disclosure of financial assets [Line Items]      
Beginning balance 805 364 276
Add: Provision for doubtful accounts 535 715 88
Deduct: Amounts written off (15) (274) 0
Ending balance 1,325 805 364
Trade notes and accounts receivable [Member] | Collectively assessed for impairment [Member]      
Disclosure of financial assets [Line Items]      
Beginning balance 968 970 773
Add: Provision for doubtful accounts 43 228 392
Deduct: Amounts written off (219) (230) (195)
Ending balance $ 792 $ 968 $ 970