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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, 2017
TWD ($)
shares
Dec. 31, 2017
USD ($)
shares
Dec. 31, 2016
TWD ($)
shares
Dec. 31, 2015
TWD ($)
shares
Net income used to compute the basic earnings per share        
Net income attributable to the parent $ 38,988 $ 1,315 $ 40,485 $ 42,039
Assumed conversion of all dilutive potential common stocks        
Employee stock options, employee bonus and compensation of subsidiaries | $ 0   (1) (1)
Net income used to compute the diluted earnings per share | $ $ 38,988   $ 40,484 $ 42,038
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 bonus or employee compensation 11 11 12 19
Weighted average number of common stocks used to compute the diluted earnings per share 7,768 7,768 7,769 7,776