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Retirement Benefit Plans - Summary of Sensitivity Analysis (Detail) - TWD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2020 | Dec. 31, 2019
Disclosure of sensitivity analysis for actuarial assumptions [Line Items] |  |
The expected contributions to the plan for the next year | $ 1,932 | $ 2,076
Bottom of range [Member] |  |
Disclosure of sensitivity analysis for actuarial assumptions [Line Items] |  |
The average duration of the defined benefit obligations | 6 years 4 months 24 days | 6 years 6 months
Top of range [Member] |  |
Disclosure of sensitivity analysis for actuarial assumptions [Line Items] |  |
The average duration of the defined benefit obligations | 13 years | 14 years

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- Definition The estimate of the expected contributions to be made to a defined benefit plan for the next annual reporting period. [Refer: Defined benefit plans [member]] + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Name IAS -Number 19 -IssueDate 2020-01-01 -Paragraph 147 -Subparagraph b -URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=19&code=ifrs-tx-2020-en-r&anchor=para_147_b&doctype=Standard -URIDate 2020-01-01 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Name IAS -Number 19 -IssueDate 2020-01-01 -Paragraph 148 -Subparagraph d -Clause iii -URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=19&code=ifrs-tx-2020-en-r&anchor=para_148_d_iii&doctype=Standard -URIDate 2020-01-01 + Details
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- Definition The weighted average duration of a defined benefit obligation. [Refer: Weighted average [member]] + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Name IAS -Number 19 -IssueDate 2020-01-01 -Paragraph 147 -Subparagraph c -URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=19&code=ifrs-tx-2020-en-r&anchor=para_147_c&doctype=Standard -URIDate 2020-01-01 + Details
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