Contents of Significant Accounts - Unused Tax Loss Carry-forward for Which No Deferred Income Tax Assets have been Recognized (Detail) - TWD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]
Unused tax loss carry-forward for which no deferred income tax assets have been recognized	$ 43,584,701	$ 48,296,329
1-5 years [member]
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]
Unused tax loss carry-forward for which no deferred income tax assets have been recognized	40,910,279	38,708,327
6-10 years [member]
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]
Unused tax loss carry-forward for which no deferred income tax assets have been recognized	$ 2,674,422	$ 9,588,002