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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
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Dec. 31, 2017 TWD ($) | Dec. 31, 2017 USD ($) | Dec. 31, 2016 TWD ($) | Dec. 31, 2015 TWD ($)
Disclosure of defined benefit plans [Line Items] |  |  |  |
Net defined benefit liability (asset) at beginning of period | $ 618 |  | $ 7,088 | $ 6,462
Current service cost | 2,918 |  | 2,866 | 2,884
Interest expense/interest income | (13) |  | 27 | 102
Amounts recognized in profit or loss | 2,905 |  | 2,893 | 2,986
Remeasurement on the net defined benefit liability |  |  |  |
Return on plan assets (excluding amounts included in net interest) | 193 |  | 352 | (136)
Actuarial (gain) losses recognized from changes in demographic assumptions | 15 |  | (124) | 11
Actuarial (gains) losses recognized from changes in financial assumptions |  |  | 1,715 | (1)
Actuarial losses recognized from experience adjustments | 1,816 |  | 100 | 357
Amounts recognized in other comprehensive income | 2,024 | $ 68 | 2,043 | 231
Contributions from employer | (2,635) |  | (11,235) | (2,435)
Benefits paid | 0 |  | 0 | 0
Benefits paid directly by the Company | (221) |  | (171) | (156)
Net defined benefit liability (asset) at end of period | 2,691 |  | 618 | 7,088
Present value of funded defined benefit obligation [Member] |  |  |  |
Disclosure of defined benefit plans [Line Items] |  |  |  |
Net defined benefit liability (asset) at beginning of period | 34,572 |  | 30,882 | 27,958
Current service cost | 2,918 |  | 2,866 | 2,884
Interest expense/interest income | 506 |  | 600 | 546
Amounts recognized in profit or loss | 3,424 |  | 3,466 | 3,430
Remeasurement on the net defined benefit liability |  |  |  |
Return on plan assets (excluding amounts included in net interest) | 0 |  | 0 | 0
Actuarial (gain) losses recognized from changes in demographic assumptions | 15 |  | (124) | 11
Actuarial (gains) losses recognized from changes in financial assumptions |  |  | 1,715 | (1)
Actuarial losses recognized from experience adjustments | 1,816 |  | 100 | 357
Amounts recognized in other comprehensive income | 1,831 |  | 1,691 | 367
Contributions from employer | 0 |  | 0 | 0
Benefits paid | (1,943) |  | (1,296) | (717)
Benefits paid directly by the Company | (221) |  | (171) | (156)
Net defined benefit liability (asset) at end of period | 37,663 |  | 34,572 | 30,882
Fair value of plan assets [Member] |  |  |  |
Disclosure of defined benefit plans [Line Items] |  |  |  |
Net defined benefit liability (asset) at beginning of period | 33,954 |  | 23,794 | 21,496
Current service cost | 0 |  | 0 | 0
Interest expense/interest income | 519 |  | 573 | 444
Amounts recognized in profit or loss | 519 |  | 573 | 444
Remeasurement on the net defined benefit liability |  |  |  |
Return on plan assets (excluding amounts included in net interest) | (193) |  | (352) | 136
Actuarial (gain) losses recognized from changes in demographic assumptions | 0 |  | 0 | 0
Actuarial (gains) losses recognized from changes in financial assumptions |  |  | 0 | 0
Actuarial losses recognized from experience adjustments | 0 |  | 0 | 0
Amounts recognized in other comprehensive income | (193) |  | (352) | 136
Contributions from employer | 2,635 |  | 11,235 | 2,435
Benefits paid | (1,943) |  | (1,296) | (717)
Benefits paid directly by the Company | 0 |  | 0 | 0
Net defined benefit liability (asset) at end of period | $ 34,972 |  | $ 33,954 | $ 23,794

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- Definition Actuarial gains (losses) arising from experience adjustments, net defined benefit liability (asset). + References No definition available. + Details
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- Definition Benefits paid. + 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 2016-01-13 -Paragraph 141 -Subparagraph c -Clause ii -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=19&date=2016-03-30&anchor=para_141_c_ii&doctype=Standard -URIDate 2016-03-31 + 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 2016-01-13 -Paragraph 141 -Subparagraph c -Clause iii -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=19&date=2016-03-30&anchor=para_141_c_iii&doctype=Standard -URIDate 2016-03-31 + 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 2016-01-13 -Paragraph 141 -Subparagraph f -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=19&date=2016-03-30&anchor=para_141_f&doctype=Standard -URIDate 2016-03-31 + 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 2016-01-13 -Paragraph 141 -Subparagraph a -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=19&date=2016-03-30&anchor=para_141_a&doctype=Standard -URIDate 2016-03-31 + 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 2016-01-13 -Paragraph 141 -Subparagraph b -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=19&date=2016-03-30&anchor=para_141_b&doctype=Standard -URIDate 2016-03-31 + 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 2016-01-13 -Paragraph 140 -Subparagraph a -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=19&date=2016-03-30&anchor=para_140_a&doctype=Standard -URIDate 2016-03-31 + 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 2016-01-13 -Paragraph 91 -Subparagraph b -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=1&date=2016-03-30&anchor=para_91_b&doctype=Standard -URIDate 2016-03-31 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Name IAS -Number 1 -IssueDate 2016-01-13 -Paragraph 7 -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=1&date=2016-03-30&anchor=para_7&doctype=Standard -URIDate 2016-03-31 + Details
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- Definition The decrease (increase) in the net defined benefit liability (asset) resulting from the return on plan assets. 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]] + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Name IAS -Number 19 -IssueDate 2016-01-13 -Paragraph 141 -Subparagraph c -Clause i -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=19&date=2016-03-30&anchor=para_141_c_i&doctype=Standard -URIDate 2016-03-31 + Details
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