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Financial Risk Management and Fair Values of Financial Instruments - Summary of Loss Rate Methodology (Detail) - TWD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Contract assets [member]    
Disclosure of detailed information about financial instruments [Line Items]    
Expected loss rate 0.03% 0.045%
Total carrying amount $ 377,983 $ 299,970
Loss allowance $ (114) $ (135)
Accounts and notes receivable including related parties [member]    
Disclosure of detailed information about financial instruments [Line Items]    
Expected loss rate 0.03% 0.045%
Total carrying amount $ 4,456,065 $ 4,749,569
Loss allowance $ (1,351) $ (2,141)
Other receivable including related parties [member]    
Disclosure of detailed information about financial instruments [Line Items]    
Expected loss rate 0.03% 0.045%
Total carrying amount $ 92,642 $ 66,181
Loss allowance $ (18) $ (13)