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Financial Instruments - Assets and Liabilities related to Foreign Currency Risk (Detail) - Foreign currency risk [Member] - TWD ($)
$ in Millions
Dec. 31, 2017
Dec. 31, 2016
USD [Member]    
Disclosure of detailed information about financial instruments [Line Items]    
Foreign currency denominated monetary assets $ 5,584 $ 5,327
Foreign currency denominated monetary liabilities 4,964 4,238
EUR [Member]    
Disclosure of detailed information about financial instruments [Line Items]    
Foreign currency denominated monetary assets 28 14
Foreign currency denominated monetary liabilities 1,323 968
Derivatives liabilities 1 2
SGD [Member]    
Disclosure of detailed information about financial instruments [Line Items]    
Foreign currency denominated monetary assets 63 106
Foreign currency denominated monetary liabilities 96 1
RMB [Member]    
Disclosure of detailed information about financial instruments [Line Items]    
Foreign currency denominated monetary assets 3 30
JPY [Member]    
Disclosure of detailed information about financial instruments [Line Items]    
Foreign currency denominated monetary assets 36 13
Foreign currency denominated monetary liabilities $ 12 $ 10