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Significant Assets and Liabilities Denominated in Foreign Currencies - Summary of Realized and Unrealized Foreign Exchange Gain (Loss) (Detail)
$ in Thousands, $ in Thousands
 12 Months Ended
Dec. 31, 2020
TWD ($)
Dec. 31, 2020
USD ($)
Dec. 31, 2019
TWD ($)
Dec. 31, 2018
TWD ($)
Disclosure of detailed information about financial instruments [line items]        
Net Foreign Exchange Gain (Loss) $ 1,056,639 $ 37,630 $ 1,133,701 $ (1,036,715)
US Dollar [member] | US dollar equals to New Taiwan dollar [member]        
Disclosure of detailed information about financial instruments [line items]        
Exchange Rate US$1=NT$28.48 US$1=NT$28.48 US$1=NT$29.98 US$1=NT$30.715
Net Foreign Exchange Gain (Loss) $ (143,804) $ (5,121) $ (84,177) $ (67,476)
New Taiwan Dollar [member]        
Disclosure of detailed information about financial instruments [line items]        
Net Foreign Exchange Gain (Loss) $ 1,382,067 $ 49,219 $ 1,203,823 $ (849,234)
Chinese Yuan [member] | Chinese Yuan equals to New Taiwan dollar [member]        
Disclosure of detailed information about financial instruments [line items]        
Exchange Rate RMB1=NT$4.3648 RMB1=NT$4.3648 RMB1=NT$4.2975 RMB1=NT$4.4753
Net Foreign Exchange Gain (Loss) $ (181,624) $ (6,468) $ 14,055 $ (120,005)