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Financial Instruments - Financing Facilities (Detail) - TWD ($)
$ in Millions
Dec. 31, 2017
Dec. 31, 2016
Unsecured loans [Member]    
Disclosure of detailed information about financial instruments [Line Items]    
Unsecured bank loan facility, amount used $ 90 $ 118
Bank loan facility, amount unused 45,749 46,219
Unsecured bank loan facility 45,839 46,337
Secured loans [Member]    
Disclosure of detailed information about financial instruments [Line Items]    
Secured loans 1,600 1,620
Bank loan facility, amount unused 1,910 200
Secured bank loan facility $ 3,510 $ 1,820