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Equity - Unrealized Gain or Loss on Financial Assets at FVOCI (Detail) - TWD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Statement Of Changes In Equity [Abstract]      
Beginning balance $ 735 $ 538 $ 883
Unrealized gain or loss for the year      
Equity instruments 522 197 (345)
Transferred accumulated gain or loss to unappropriated earnings resulting from the disposal of equity instruments (17) 0 0
Ending balance $ 1,240 $ 735 $ 538