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Financial Risk and Fair Value Disclosures - Additional Information (Detail)
$ in Thousands, ¥ in Billions
12 Months Ended
Oct. 01, 2019
TWD ($)
Dec. 31, 2020
TWD ($)
Dec. 31, 2019
TWD ($)
Dec. 31, 2018
TWD ($)
Dec. 31, 2018
JPY (¥)
Disclosure of nature and extent of risks arising from financial instruments [line items]          
Financial assets   $ 166,250,375 $ 155,644,299    
Accounts receivables from the top ten customers   62.00% 44.00%    
Profit or loss attributable to change in unrealized gains or losses for financial assets without quoted market prices held at end of period   $ 721,000 $ (113,000) $ (203,000)  
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%
Increase (decrease) in profit due to percentage increase in market risk   $ 44,000 $ 46,000 44,000  
Percentage decrease in market risk   0.10% 0.10%   0.10%
Increase (decrease) in profit due to percentage decrease in market risk   $ (44,000) $ (46,000) (44,000)  
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%
Increase (decrease) in profit due to percentage increase in market risk   $ 263,000 $ 252,000 171,000  
Percentage decrease in market risk   5.00% 5.00%   5.00%
Increase (decrease) in profit due to percentage decrease in market risk   $ (263,000) $ (252,000) (171,000)  
Increase (decrease) in other comprehensive income due to percentage increase in market risk   453,000 671,000 408,000  
Increase (decrease) in other comprehensive income due to percentage decrease in market risk   $ (453,000) $ (671,000) (408,000)  
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%
Increase (decrease) in profit due to percentage increase in market risk   $ 1,336,000 $ 1,009,000 1,367,000  
Percentage decrease in market risk   10.00% 10.00%   10.00%
Increase (decrease) in profit due to percentage decrease in market risk   $ (1,336,000) $ (1,009,000) (1,367,000)  
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%
Increase (decrease) in profit due to percentage increase in market risk   $ 1,625,000 $ 2,200,000 2,624,000  
Percentage decrease in market risk   10.00% 10.00%   10.00%
Increase (decrease) in profit due to percentage decrease in market risk   $ (1,625,000) $ (2,200,000) (2,624,000)  
Japan, Yen | 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%
Increase (decrease) in profit due to percentage increase in market risk   $ 473,000 $ 430,000 4,000  
Percentage decrease in market risk   10.00% 10.00%   10.00%
Increase (decrease) in profit due to percentage decrease in market risk   $ (473,000) $ (430,000) $ (4,000)  
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)        
Ownership interest 84.10%