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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, 2017
TWD ($)
Dec. 31, 2017
USD ($)
Dec. 31, 2016
TWD ($)
Dec. 31, 2015
TWD ($)
Disclosure of detailed information about financial instruments [line items]        
Net Foreign Exchange Gain (Loss) $ 3,547,811 $ 119,697 $ 1,921,695 $ (830,073)
New Taiwan Dollar [member]        
Disclosure of detailed information about financial instruments [line items]        
Net Foreign Exchange Gain (Loss) $ 4,130,243 $ 139,347 $ 1,494,044 $ (695,510)
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$29.76 US$1=NT$29.76 US$1=NT$32.25 US$1=NT$32.825
Net Foreign Exchange Gain (Loss) $ (244,802) $ (8,259) $ 203,258 $ 136,795
Chinese Yuan [member] | Chinese Yuan equals to New Taiwan dollar [member]        
Disclosure of detailed information about financial instruments [line items]        
Exchange Rate CNY1=NT$4.5545 CNY1=NT$4.5545 CNY1=NT$4.649 CNY1=NT$5.0550
Net Foreign Exchange Gain (Loss) $ (337,630) $ (11,391) $ 224,393 $ (271,358)