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Earnings Per Share - Net Income and Weighted Average Number of Common Stocks Used in Calculation of Earnings Per Share (Detail)
shares in Millions, $ in Millions, $ in Millions
 12 Months Ended
Dec. 31, 2020
TWD ($)
shares
Dec. 31, 2020
USD ($)
shares
Dec. 31, 2019
TWD ($)
shares
Dec. 31, 2018
TWD ($)
shares
Net income used to compute the basic earnings per share        
Net income attributable to the parent $ 33,419 $ 1,190 $ 32,947 $ 37,557
Assumed conversion of all dilutive potential common stocks        
Employee stock options, employee compensation of subsidiaries | $ (7)   (4) (6)
Net income used to compute the diluted earnings per share | $ $ 33,412   $ 32,943 $ 37,551
Weighted average number of common stocks used to compute the basic earnings per share 7,757 7,757 7,757 7,757
Assumed conversion of all dilutive potential common stocks        
Employee compensation 8 8 8 9
Weighted average number of common stocks used to compute the diluted earnings per share 7,765 7,765 7,765 7,766