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Earnings Per Share - Summary of Earnings and Weighted Average Number of Ordinary Shares Outstanding in Computation of Earnings Per Share (Detail)
shares in Thousands, $ in Thousands, $ in Thousands
 12 Months Ended
Dec. 31, 2020
TWD ($)
shares
Dec. 31, 2020
USD ($)
shares
Dec. 31, 2019
TWD ($)
shares
Dec. 31, 2018
TWD ($)
shares
Earnings per share [line items]        
Profit for the year attributable to owners of the Company $ 26,970,580 $ 960,490 $ 17,060,591 $ 26,220,721
Effect of potentially dilutive ordinary shares:        
Employee share options issued by subsidiaries (521,073) (18,557) (385,865) (418,295)
Earnings used in the computation of diluted earnings per share $ 26,449,507 $ 941,933 $ 16,674,726 $ 25,802,426
Weighted average number of ordinary shares in the computation of basic earnings per share 4,265,732 4,265,732 4,251,964 4,245,247
Effect of potentially dilutive ordinary shares:        
From employee share options 22,086 22,086 10,232 5,103
From employees' compensation 815 815 570 779
Weighted average number of ordinary shares in computation of diluted earnings per share 4,288,633 4,288,633 4,262,766 4,251,129