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Earnings Per Share - Computation of Earnings Per Share (Detail)
$ / shares in Units, shares in Millions, $ in Millions
 12 Months Ended
Dec. 31, 2017
TWD ($)
$ / shares
shares
Dec. 31, 2017
$ / shares
Dec. 31, 2016
TWD ($)
$ / shares
shares
Dec. 31, 2015
TWD ($)
$ / shares
shares
Earnings per share [abstract]        
Net income available to common shareholders of the parent | $ $ 344,998.3   $ 331,713.7 $ 302,850.9
Effect of dilutive potential common shares | $       0.0
Net income available to common shareholders of the parent (including effect of dilutive potential common shares) | $       $ 302,850.9
Net income available to common shareholders of the parent | shares     25,930.3 25,930.3
Effect of dilutive potential common shares | shares       0.1
Net income available to common shareholders of the parent | shares 25,930.3     25,930.4
Basic EPS | (per share) $ 13.30 $ 0.45 $ 12.79 $ 11.68
Diluted EPS | (per share) $ 13.30 $ 0.45 $ 12.79 $ 11.68