Earnings Per Share - Income and Share Data Used in Basic and Diluted EPS Computations (Detail) $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
Dec. 31, 2020 TWD ($) $ / shares shares	Dec. 31, 2020 $ / shares	Dec. 31, 2019 TWD ($) $ / shares shares	Dec. 31, 2018 TWD ($) $ / shares shares
Basic earnings per share, amount after tax
Equity holders of the Company - Amount after tax \| $	$ 2,378,978		$ 2,508,574	$ 1,325,824
Diluted earnings per share, amount after tax
Equity holders of the Company - Amount after tax \| $	$ 2,378,978		$ 2,508,574	$ 1,325,824
Weighted average number of ordinary shares outstanding	727,240		727,111	802,725
Diluted earnings per share, weighted-average number of ordinary shares outstanding
Employees' compensations	9,668		9,879	7,626
Restricted shares			126	3,356
Weighted average number of ordinary shares outstanding	736,908		737,116	813,707
Ordinary shares [member]
Basic earnings per share
Equity holders of the Company - Earnings per share \| (per share)	$ 3.27	$ 0.12	$ 3.45	$ 1.65
Diluted earnings per share
Equity holders of the Company - Earnings per share \| (per share)	$ 3.23	$ 0.11	$ 3.40	$ 1.63

The weighted average number of ordinary shares outstanding plus the weighted average number of ordinary shares that would be issued on the conversion of all the dilutive potential ordinary shares into ordinary shares. [Refer: Ordinary shares [member]; Weighted average [member]]

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The amount of profit (loss) attributable to ordinary equity holders of the parent entity (the numerator) divided by the weighted average number of ordinary shares outstanding during the period (the denominator).

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The amount of profit (loss) attributable to ordinary equity holders of the parent entity (the numerator), divided by the weighted average number of ordinary shares outstanding during the period (the denominator), both adjusted for the effects of all dilutive potential ordinary shares. [Refer: Ordinary shares [member]; Weighted average [member]]

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The profit (loss) attributable to ordinary equity holders of the parent entity. [Refer: Profit (loss)]

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The number of ordinary shares outstanding at the beginning of the period, adjusted by the number of ordinary shares bought back or issued during the period multiplied by a time-weighting factor.

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Adjusted weighted average number of employees' compensations outstanding.

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