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Retirement Benefit Plans - Movements in Defined Benefit Obligation and Fair Value of Plan Assets (Detail) $ in Millions, $ in Millions |  | 12 Months Ended
--- | --- | ---
Jul. 01, 2020 TWD ($) | Dec. 31, 2020 TWD ($) | Dec. 31, 2020 USD ($) | Dec. 31, 2019 TWD ($) | Dec. 31, 2018 TWD ($)
Disclosure Of Defined Plan [line items] |  |  |  |  |
Net defined benefit liability (asset) at beginning of period |  | $ 1,377 |  | $ 2,370 | $ 2,691
Current service cost |  | 2,052 |  | 2,927 | 3,024
Interest expense/interest income |  | 1 |  | 10 | 6
Amounts recognized in profit or loss |  | 2,053 |  | 2,937 | 3,030
Remeasurement on the net defined benefit liability |  |  |  |  |
Return on plan assets (excluding amounts included in net interest) |  | (1,308) |  | (1,338) | (875)
Actuarial (gains) losses recognized from changes in demographic assumptions |  |  |  | 6 | 4
Actuarial (gains) losses recognized from changes in financial assumptions |  | 590 |  | 647 | 1,273
Actuarial (gains) losses recognized from experience adjustments |  | (475) |  | (842) | 813
Amounts recognized in other comprehensive income |  | (1,193) | $ (43) | (1,527) | 1,215
Contributions from employer |  | (1,964) |  | (2,099) | (4,374)
Benefits paid |  | 0 |  | 0 | 0
Benefits paid directly by the Company |  | (263) |  | (304) | (192)
Acquired by business combinations (Note 14) | $ 32 |  |  |  |
Net defined benefit liability (asset) at end of period |  | 42 |  | 1,377 | 2,370
Present value of funded defined benefit obligation [Member] |  |  |  |  |
Disclosure Of Defined Plan [line items] |  |  |  |  |
Net defined benefit liability (asset) at beginning of period |  | 41,197 |  | 41,397 | 37,663
Current service cost |  | 2,052 |  | 2,927 | 3,024
Interest expense/interest income |  | 298 |  | 400 | 550
Amounts recognized in profit or loss |  | 2,350 |  | 3,327 | 3,574
Remeasurement on the net defined benefit liability |  |  |  |  |
Return on plan assets (excluding amounts included in net interest) |  | 0 |  | 0 | 0
Actuarial (gains) losses recognized from changes in demographic assumptions |  |  |  | 6 | 4
Actuarial (gains) losses recognized from changes in financial assumptions |  | 590 |  | 647 | 1,273
Actuarial (gains) losses recognized from experience adjustments |  | (475) |  | (842) | 813
Amounts recognized in other comprehensive income |  | 115 |  | (189) | 2,090
Contributions from employer |  | 0 |  | 0 | 0
Benefits paid |  | (3,919) |  | (3,034) | (1,738)
Benefits paid directly by the Company |  | (263) |  | (304) | (192)
Acquired by business combinations (Note 14) | 56 |  |  |  |
Net defined benefit liability (asset) at end of period |  | 39,536 |  | 41,197 | 41,397
Fair value of plan assets [Member] |  |  |  |  |
Disclosure Of Defined Plan [line items] |  |  |  |  |
Net defined benefit liability (asset) at beginning of period |  | 39,820 |  | 39,027 | 34,972
Current service cost |  | 0 |  | 0 | 0
Interest expense/interest income |  | 297 |  | 390 | 544
Amounts recognized in profit or loss |  | 297 |  | 390 | 544
Remeasurement on the net defined benefit liability |  |  |  |  |
Return on plan assets (excluding amounts included in net interest) |  | 1,308 |  | 1,338 | 875
Actuarial (gains) losses recognized from changes in demographic assumptions |  |  |  | 0 | 0
Actuarial (gains) losses recognized from changes in financial assumptions |  | 0 |  | 0 | 0
Actuarial (gains) losses recognized from experience adjustments |  | 0 |  | 0 | 0
Amounts recognized in other comprehensive income |  | 1,308 |  | 1,338 | 875
Contributions from employer |  | 1,964 |  | 2,099 | 4,374
Benefits paid |  | (3,919) |  | (3,034) | (1,738)
Benefits paid directly by the Company |  | 0 |  | 0 | 0
Acquired by business combinations (Note 14) | $ 24 |  |  |  |
Net defined benefit liability (asset) at end of period |  | $ 39,494 |  | $ 39,820 | $ 39,027

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- Definition Benefits paid directly by employer. + References No definition available. + Details
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- Definition Defined benefit cost recognized in profit or loss. + References No definition available. + Details
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- Definition The decrease (increase) in a net defined benefit liability (asset) resulting from actuarial gains (losses) arising from changes in demographic assumptions that result in remeasurements of the net defined benefit liability (asset). Demographic assumptions deal with matters such as: (a) mortality; (b) rates of employee turnover, disability and early retirement; (c) the proportion of plan members with dependants who will be eligible for benefits; (d) the proportion of plan members who will select each form of payment option available under the plan terms; and (e) claim rates under medical plans. [Refer: Net defined benefit liability (asset)] + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Name IAS -Number 19 -IssueDate 2020-01-01 -Paragraph 141 -Subparagraph c -Clause ii -URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=19&code=ifrs-tx-2020-en-r&anchor=para_141_c_ii&doctype=Standard -URIDate 2020-01-01 + Details
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- Definition The decrease (increase) in a net defined benefit liability (asset) resulting from actuarial gains (losses) arising from changes in financial assumptions that result in remeasurements of the net defined benefit liability (asset). Financial assumptions deal with items such as: (a) the discount rate; (b) benefit levels, excluding any cost of the benefits to be met by employees, and future salary; (c) in the case of medical benefits, future medical costs, including claim handling costs (ie the costs that will be incurred in processing and resolving claims, including legal and adjuster's fees); and (d) taxes payable by the plan on contributions relating to service before the reporting date or on benefits resulting from that service. [Refer: Net defined benefit liability (asset)] + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Name IAS -Number 19 -IssueDate 2020-01-01 -Paragraph 141 -Subparagraph c -Clause iii -URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=19&code=ifrs-tx-2020-en-r&anchor=para_141_c_iii&doctype=Standard -URIDate 2020-01-01 + Details
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- Definition The decrease (increase) in a net defined benefit liability (asset) resulting from actuarial gains (losses) arising from experience adjustments that result in remeasurements of the net defined benefit liability (asset). Experience adjustments deal with the effects of differences between the previous actuarial assumptions and what has actually occurred. [Refer: Net defined benefit liability (asset)] + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Name IAS -Number 19 -IssueDate 2020-01-01 -Paragraph 141 -Subparagraph c -URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=19&code=ifrs-tx-2020-en-r&anchor=para_141_c&doctype=Standard -URIDate 2020-01-01 + Details
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- Definition The decrease (increase) in net defined benefit liability (asset) resulting from contributions to a defined benefit plan by the employer. [Refer: Net defined benefit liability (asset); Defined benefit plans [member]] + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Name IAS -Number 19 -IssueDate 2020-01-01 -Paragraph 141 -Subparagraph f -URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=19&code=ifrs-tx-2020-en-r&anchor=para_141_f&doctype=Standard -URIDate 2020-01-01 + Details
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- Definition The increase (decrease) in the net defined benefit liability (asset) resulting from employee service in the current period. [Refer: Net defined benefit liability (asset)] + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Name IAS -Number 19 -IssueDate 2020-01-01 -Paragraph 141 -Subparagraph a -URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=19&code=ifrs-tx-2020-en-r&anchor=para_141_a&doctype=Standard -URIDate 2020-01-01 + Details
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- Definition Line items represent concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes of the table. + References No definition available. + Details
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- Definition The increase (decrease) in the net defined benefit liability (asset) resulting from business combinations and disposals. [Refer: Business combinations [member]; Net defined benefit liability (asset)] + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Name IAS -Number 19 -IssueDate 2020-01-01 -Paragraph 141 -Subparagraph h -URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=19&code=ifrs-tx-2020-en-r&anchor=para_141_h&doctype=Standard -URIDate 2020-01-01 + Details
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- Definition The increase (decrease) in the net defined benefit liability (asset) resulting from the passage of time. [Refer: Interest expense; Net defined benefit liability (asset)] + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Name IAS -Number 19 -IssueDate 2020-01-01 -Paragraph 141 -Subparagraph b -URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=19&code=ifrs-tx-2020-en-r&anchor=para_141_b&doctype=Standard -URIDate 2020-01-01 + Details
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- Definition The amount of deficit or surplus in a defined benefit plan, adjusted for any effect of limiting a net defined benefit asset to the asset ceiling. [Refer: Defined benefit plans [member]] + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Name IAS -Number 19 -IssueDate 2020-01-01 -Paragraph 140 -Subparagraph a -URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=19&code=ifrs-tx-2020-en-r&anchor=para_140_a&doctype=Standard -URIDate 2020-01-01 + Details
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- Definition The amount of other comprehensive income, before tax, related to gains (losses) on remeasurements of defined benefit plans, which comprise actuarial gains and losses; the return on plan assets, excluding amounts included in net interest on the net defined benefit liability (asset); and any change in the effect of the asset ceiling, excluding amounts included in net interest on the net defined benefit liability (asset). [Refer: Other comprehensive income, before tax; Defined benefit plans [member]; Plan assets [member]; Net defined benefit liability (asset)] + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Name IAS -Number 1 -IssueDate 2020-01-01 -Paragraph 91 -Subparagraph b -URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=1&code=ifrs-tx-2020-en-r&anchor=para_91_b&doctype=Standard -URIDate 2020-01-01 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Name IAS -Number 1 -IssueDate 2020-01-01 -Paragraph 7 -URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=1&code=ifrs-tx-2020-en-r&anchor=para_7&doctype=Standard -URIDate 2020-01-01 + Details
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- Definition The decrease (increase) in the net defined benefit liability (asset) resulting from the return on plan assets, excluding amounts included in interest income or expense. The return on plans assets is interest, dividends and other revenue derived from the plan assets, together with realised and unrealised gains or losses on the plan assets, less any costs of managing plan assets and less any tax payable by the plan itself, other than tax included in the actuarial assumptions used to measure the present value of the defined benefit obligation. [Refer: Plan assets [member]; Net defined benefit liability (asset); Actuarial assumptions [member]; Interest expense (income), net defined benefit liability (asset)] + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Name IAS -Number 19 -IssueDate 2020-01-01 -Paragraph 141 -Subparagraph c -Clause i -URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=19&code=ifrs-tx-2020-en-r&anchor=para_141_c_i&doctype=Standard -URIDate 2020-01-01 + Details
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