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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, 2018
USD ($)
Dec. 31, 2018
TWD ($)
Dec. 31, 2017
TWD ($)
Dec. 31, 2016
TWD ($)
Disclosure of detailed information about financial instruments [line items]        
Net Foreign Exchange Gain (Loss) $ (33,868) $ (1,036,715) $ 3,547,811 $ 1,921,695
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$30.715 US$1=NT$30.715 US$1=NT$29.76 US$1=NT$32.25
Net Foreign Exchange Gain (Loss) $ (2,204) $ (67,476) $ (244,802) $ 203,258
New Taiwan Dollar [member]        
Disclosure of detailed information about financial instruments [line items]        
Net Foreign Exchange Gain (Loss) $ (27,744) $ (849,234) $ 4,130,243 $ 1,494,044
Chinese Yuan [member] | Chinese Yuan equals to New Taiwan dollar [member]        
Disclosure of detailed information about financial instruments [line items]        
Exchange Rate CNY1=NT$4.4753 CNY1=NT$4.4753 CNY1=NT$4.5545 CNY1=NT$4.649
Net Foreign Exchange Gain (Loss) $ (3,920) $ (120,005) $ (337,630) $ 224,393