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Financial Risk and Fair Value Disclosures - Additional Information (Detail)
$ in Thousands, $ in Thousands, ¥ in Billions
12 Months Ended
Dec. 31, 2019
TWD ($)
Dec. 31, 2018
TWD ($)
Dec. 31, 2017
TWD ($)
Dec. 31, 2019
USD ($)
Dec. 31, 2018
JPY (¥)
Disclosure of nature and extent of risks arising from financial instruments [line items]          
Financial assets $ 155,644,299 $ 136,986,309      
Cash flow hedge reserve in other components of equity $ 0 $ (2,058)   $ 0  
Accounts receivables from the top ten customers 44.00% 54.00%      
Profit or loss attributable to change in unrealized gains or losses for financial assets without quoted market prices held at end of period $ 113,000 $ 203,000 $ 286,000    
Interest rate risk [member]          
Disclosure of nature and extent of risks arising from financial instruments [line items]          
Percentage increase in market risk 0.10% 0.10% 0.10% 0.10% 0.10%
Increase (decrease) in profit due to percentage increase in market risk $ (46,000) $ (44,000) $ (58,000)    
Percentage decrease in market risk 0.10% 0.10% 0.10% 0.10% 0.10%
Increase (decrease) in profit due to percentage decrease in market risk $ 46,000 $ 44,000 $ 58,000    
Equity price risk [member]          
Disclosure of nature and extent of risks arising from financial instruments [line items]          
Percentage increase in market risk 5.00% 5.00% 5.00% 5.00% 5.00%
Increase (decrease) in profit due to percentage increase in market risk $ 252,000 $ 171,000 $ 33,000    
Percentage decrease in market risk 5.00% 5.00% 5.00% 5.00% 5.00%
Increase (decrease) in profit due to percentage decrease in market risk $ (252,000) $ (171,000) $ (33,000)    
Increase (decrease) in other comprehensive income due to percentage increase in market risk 671,000 408,000 979,000    
Increase (decrease) in other comprehensive income due to percentage decrease in market risk $ (671,000) $ (408,000) $ (979,000)    
Taiwan, New Dollars | Currency risk [member]          
Disclosure of nature and extent of risks arising from financial instruments [line items]          
Percentage increase in market risk 10.00% 10.00% 10.00% 10.00% 10.00%
Increase (decrease) in profit due to percentage increase in market risk $ (1,009,000) $ (1,367,000) $ (1,330,000)    
Percentage decrease in market risk 10.00% 10.00% 10.00% 10.00% 10.00%
Increase (decrease) in profit due to percentage decrease in market risk $ 1,009,000 $ 1,367,000 $ 1,330,000    
China, Yuan Renminbi | Currency risk [member]          
Disclosure of nature and extent of risks arising from financial instruments [line items]          
Percentage increase in market risk 10.00% 10.00% 10.00% 10.00% 10.00%
Increase (decrease) in profit due to percentage increase in market risk $ 2,200,000 $ 2,624,000 $ 4,011,000    
Percentage decrease in market risk 10.00% 10.00% 10.00% 10.00% 10.00%
Increase (decrease) in profit due to percentage decrease in market risk $ (2,200,000) $ (2,624,000) $ (4,011,000)    
Forward exchange contracts [member]          
Disclosure of nature and extent of risks arising from financial instruments [line items]          
Financial assets | ¥         ¥ 23
Cash flow hedge reserve in other components of equity $ (3,000)