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Fair Value Information - Schedule of Impact of Decrease in Discount for Lack of Marketability or Noncontrolling Interests (Detail) - TWD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2020 | Dec. 31, 2019
Disclosure Of Fair Value Measurement Of Financial Instruments [Abstract] |  |
Discount for lack of marketability 5% decrease | $ 320 | $ 342
Noncontrolling interests discount 5% decrease | $ 47 | $ 54

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