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Financial Risk Management and Fair Values of Financial Instruments - Additional Information (Detail) - TWD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Disclosure of detailed information about financial instruments [Line Items]      
Realized and unrealized gain and loss arising from significant foreign exchange variation $ (355,255) $ (154,993) $ 93,104
General increase or decrease of 100 basis points (1%) in interest rates [member]      
Disclosure of detailed information about financial instruments [Line Items]      
Estimated decrease or increase in the Group's profit and equity based on an increase or decrease in the interest rates 78,150 90,660 98,220
Equity price risk [member]      
Disclosure of detailed information about financial instruments [Line Items]      
Estimated decrease or increase in the Group's profit and equity based on an increase or decrease in the interest rates $ 531 $ 0 $ 0
Adjustments to interest rate   1.00%