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Short Term Loans - Summary of Short-term Loans (Detail) - Short-term loans [Member] - TWD ($)
$ in Millions
Dec. 31, 2020
Dec. 31, 2019
Disclosure of detailed information about borrowings [Line Items]    
Unsecured bank loans $ 67 $ 90
Unsecured loans [Member] | Bottom of Range [Member]    
Disclosure of detailed information about borrowings [Line Items]    
Interest rates of bank loans 1.12% 1.20%
Unsecured loans [Member] | Top of Range [Member]    
Disclosure of detailed information about borrowings [Line Items]    
Interest rates of bank loans 2.33% 2.50%