Contents of Significant Accounts - Income Tax - Additional Information (Detail) - TWD ($) $ in Millions	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Disclosure Of Temporary Difference Unused Tax Losses And Unused Tax Credits [Abstract]
Deductible temporary differences for which no deferred income tax assets have been recognized	$ 2,942	$ 2,770
Taxable temporary differences of unrecognized deferred tax liabilities associated with investments in subsidiaries	$ 11,463	$ 11,389
Corporate income tax rate	20.00%	20.00%	20.00%	17.00%
Undistributed earnings tax	5.00%	5.00%	5.00%	10.00%

The aggregate amount of temporary differences associated with investments in subsidiaries, branches and associates and interests in joint arrangements, for which deferred tax liabilities have not been recognised as a result of satisfying both of the following conditions: (a) the parent, investor, joint venturer or joint operator is able to control the timing of the reversal of the temporary difference; and (b) it is probable that the temporary difference will not reverse in the foreseeable future. [Refer: Associates [member]; Subsidiaries [member]; Temporary differences [member]; Investments in subsidiaries]

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 12
-IssueDate 2020-01-01
-Paragraph 81
-Subparagraph f
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=12&code=ifrs-tx-2020-en-r&anchor=para_81_f&doctype=Standard
-URIDate 2020-01-01

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Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	na
Period Type:	instant