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Hedging Financial Instruments - Summary of Outstanding Forward Exchange Contracts (Detail) - Hedging instruments [member]
€ in Millions, $ in Millions
12 Months Ended
Dec. 31, 2017
TWD ($)
Dec. 31, 2018
TWD ($)
Dec. 31, 2017
EUR (€)
Financial liabilities- current      
Financial liabilities current   $ 155.8  
Sell NT$/Buy EUR [member] | Bottom of range [member]      
Disclosure of detailed information about financial instruments [line items]      
Maturity Date 2018-02    
Sell NT$/Buy EUR [member] | Top of range [member]      
Disclosure of detailed information about financial instruments [line items]      
Maturity Date 2018-05    
Sell [member] | Sell NT$/Buy EUR [member]      
Disclosure of detailed information about financial instruments [line items]      
Contract Amount $ 2,649.1    
Buy [member] | Sell NT$/Buy EUR [member]      
Disclosure of detailed information about financial instruments [line items]      
Contract Amount | €     € 75.0
Cash flow hedges [member] | Forward exchange contracts [member]      
Financial assets- current      
Financial assets current   23.5  
Financial liabilities- current      
Financial liabilities current   1.9  
Fair value hedges [member] | Interest rate futures contracts [member]      
Financial liabilities- current      
Financial liabilities current   $ 153.9