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Short-term Loans - Summary of Short-term Loans (Detail)
$ in Millions, $ in Millions
 12 Months Ended
Dec. 31, 2017
TWD ($)
Dec. 31, 2016
TWD ($)
Dec. 31, 2017
USD ($)
Dec. 31, 2016
USD ($)
Disclosure of detailed information about borrowings [line items]        
Amount $ 63,766.8 $ 57,958.2 $ 2,151.4  
Original loan content, US$     $ 2,150.0 $ 1,800.0
Maturity date 2018-02 2017-01    
Bottom of range [member]        
Disclosure of detailed information about borrowings [line items]        
Annual interest rate 1.54% 0.87% 1.54% 0.87%
Top of range [member]        
Disclosure of detailed information about borrowings [line items]        
Annual interest rate 1.82% 1.07% 1.82% 1.07%
Unsecured Loans [Member]        
Disclosure of detailed information about borrowings [line items]        
Amount $ 63,766.8 $ 57,958.2