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Hedging Financial Instruments - Summary of Hedging Derivatives Financial Instruments (Detail)
$ in Millions
Dec. 31, 2017
TWD ($)
Disclosure of detailed information about hedging instruments [line items]  
Hedging derivative financial instruments, assets $ 34.4
Interest rate futures contracts [member] | Fair value hedges [member]  
Disclosure of detailed information about hedging instruments [line items]  
Hedging derivative financial instruments, assets 27.0
Forward exchange contracts [member] | Cash flow hedges [member]  
Disclosure of detailed information about hedging instruments [line items]  
Hedging derivative financial instruments, assets 7.4
Hedging derivative financial instruments, liabilities $ 15.6