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Equity - Additional Information (Detail)
$ / shares in Units, $ in Thousands
 12 Months Ended
Dec. 31, 2019
TWD ($)
shares
Dec. 31, 2018
TWD ($)
$ / shares
shares
Disclosure of classes of share capital [line items]    
Number of shares represented by each ADS 2  
Percentage of legal reserve on share capital   25.00%
Cash dividends paid from the capital surplus | $ $ 10,795,980 $ (10,795,980)
American depository shares (ADs) [member]    
Disclosure of classes of share capital [line items]    
Number of shares represented by each ADS 2  
Number of shares outstanding 125,542,000 140,042,000
Ordinary shares [member]    
Disclosure of classes of share capital [line items]    
Number of shares outstanding 251,084,000 280,085,000
A.S.E. Holding Limited [member]    
Disclosure of classes of share capital [line items]    
Repurchase of ordinary shares   1,852,000
Repurchase of ordinary shares, price per share | $ / shares   $ 38.5
Treasury shares canceled   121,852,000
Top of range [member]    
Disclosure of classes of share capital [line items]    
Percentage of cash dividend on dividend distribution   30.00%