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Financial Instruments - Foreign Currency Risk Sensitivity Analysis (Detail) - TWD ($)
$ in Millions
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Profit and loss [Member] | Financial instruments [Member] | USD [Member]      
Disclosure of detailed information about financial instruments [Line Items]      
Sensitivity analysis due to a 5% increase or decrease in foreign currency exchange $ 31 $ 54 $ 21
Profit and loss [Member] | Financial instruments [Member] | EUR [Member]      
Disclosure of detailed information about financial instruments [Line Items]      
Sensitivity analysis due to a 5% increase or decrease in foreign currency exchange (65) (48) (62)
Profit and loss [Member] | Financial instruments [Member] | SGD [Member]      
Disclosure of detailed information about financial instruments [Line Items]      
Sensitivity analysis due to a 5% increase or decrease in foreign currency exchange (2) 5 5
Profit and loss [Member] | Financial instruments [Member] | RMB [Member]      
Disclosure of detailed information about financial instruments [Line Items]      
Sensitivity analysis due to a 5% increase or decrease in foreign currency exchange 0 1 2
Profit and loss [Member] | Financial instruments [Member] | JPY [Member]      
Disclosure of detailed information about financial instruments [Line Items]      
Sensitivity analysis due to a 5% increase or decrease in foreign currency exchange 1 0 12
Profit and loss [Member] | Derivatives [Member] | USD [Member]      
Disclosure of detailed information about financial instruments [Line Items]      
Sensitivity analysis due to a 5% increase or decrease in foreign currency exchange 6 3 1
Profit and loss [Member] | Derivatives [Member] | EUR [Member]      
Disclosure of detailed information about financial instruments [Line Items]      
Sensitivity analysis due to a 5% increase or decrease in foreign currency exchange 3 8 33
Equity [member] | Derivatives [Member] | EUR [Member]      
Disclosure of detailed information about financial instruments [Line Items]      
Sensitivity analysis due to a 5% increase or decrease in foreign currency exchange $ 7 $ 5 $ 15