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Equity - Summary of Unrealized Gain (Loss) on Financial Assets at FVTOCI (Detail)
$ in Thousands, $ in Thousands
 12 Months Ended
Dec. 31, 2020
TWD ($)
Dec. 31, 2020
USD ($)
Dec. 31, 2019
TWD ($)
Financial assets at fair value through other comprehensive income [line items]      
Balance at January 1 $ 1,770,775    
Ending balance 1,741,134 $ 62,006 $ 1,770,775
Unrealized gain (loss) on financial assets at fair value through other comprehensive income [member]      
Financial assets at fair value through other comprehensive income [line items]      
Balance at January 1 (203,098) (7,233) (1,015,107)
Debt instruments (2,136) (76) (2,052)
Equity instruments (405,020) (14,424) (283,472)
Share from associates and joint venture accounted for using the equity method 2,655,570 94,572 1,501,689
Share from the disposal of associates and joint venture accounted for using the equity method 1,094 39  
Equity instruments 16,383 583  
Share from associates and joint venture accounted for using the equity method (34,891) (1,242) (404,156)
Ending balance $ 2,027,902 $ 72,219 $ (203,098)