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Short-term Loans - Summary of Short-term Loans (Detail)
€ in Millions, $ in Millions, $ in Millions
 12 Months Ended
Dec. 31, 2018
TWD ($)
Dec. 31, 2017
TWD ($)
Dec. 31, 2018
USD ($)
Dec. 31, 2018
EUR (€)
Dec. 31, 2017
USD ($)
Disclosure of detailed information about borrowings [line items]          
Amount $ 88,754.7 $ 63,766.8 $ 2,899.5    
Original loan content     $ 2,610.0 € 242.0 $ 2,150.0
Maturity date 2019-01 2018-02      
Bottom of range [member]          
Disclosure of detailed information about borrowings [line items]          
Annual interest rate 0.01% 1.54% 0.01% 0.01% 1.54%
Top of range [member]          
Disclosure of detailed information about borrowings [line items]          
Annual interest rate 3.22% 1.82% 3.22% 3.22% 1.82%
Unsecured Loans [member]          
Disclosure of detailed information about borrowings [line items]          
Amount $ 88,754.7 $ 63,766.8