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Retirement Benefit Plans - Summary of Movements in Net Defined Benefit Liability (Asset) (Detail) $ in Thousands, $ in Thousands | 12 Months Ended
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Dec. 31, 2018 USD ($) | Dec. 31, 2018 TWD ($) | Dec. 31, 2017 TWD ($) | Dec. 31, 2016 TWD ($)
Disclosure of net defined benefit liability (asset) [line items] |  |  |  |
Beginning balance | $ 116,604 | $ 3,569,265 | $ 3,972,517 | $ 3,999,947
Current service cost | 7,322 | 224,126 | 278,412 | 329,838
Past service cost and gain on settlements |  |  | (68,979) |
Net interest expense (income) | 1,832 | 56,070 | 53,663 | 58,031
Recognized in profit or loss | 9,154 | 280,196 | 263,096 | 387,869
Return on plan assets (excluding amounts included in net interest) | (542) | (16,589) | 52,124 | 54,549
Actuarial loss (gain) arising from changes in financial assumptions | (282) | (8,643) | 56,860 | 156,193
Actuarial loss (gain) arising from experience adjustments | 9,882 | 302,499 | (315,090) | 200,723
Actuarial loss arising from changes in demographic assumptions | 268 | 8,190 | 762 | 5,716
Actuarial gain arising from changes in other assumptions | 742 | 22,723 |  |
Recognized in other comprehensive income | 10,068 | 308,180 | (205,344) | 417,181
Contributions from the employer | (11,899) | (364,237) | (484,790) | (807,232)
Benefits paid from the Group | (9,669) | (295,953) | (96,575) | (36,033)
Business combinations | 42,871 | 1,312,281 |  |
Exchange differences on foreign plans | (154) | (4,716) | 120,361 | 10,785
Ending balance | 156,975 | 4,805,016 | 3,569,265 | 3,972,517
Defined benefit obligation [member] |  |  |  |
Disclosure of net defined benefit liability (asset) [line items] |  |  |  |
Beginning balance | 258,433 | 7,910,638 | 8,389,884 | 7,973,676
Current service cost | 7,322 | 224,126 | 278,412 | 329,838
Past service cost and gain on settlements |  |  | (68,979) |
Net interest expense (income) | 5,841 | 178,779 | 157,404 | 167,111
Recognized in profit or loss | 13,163 | 402,905 | 366,837 | 496,949
Actuarial loss (gain) arising from changes in financial assumptions | (282) | (8,643) | 56,860 | 156,193
Actuarial loss (gain) arising from experience adjustments | 9,882 | 302,499 | (315,090) | 200,723
Actuarial loss arising from changes in demographic assumptions | 268 | 8,190 | 762 | 5,716
Actuarial gain arising from changes in other assumptions | 742 | 22,723 |  |
Recognized in other comprehensive income | 10,610 | 324,769 | (257,468) | 362,632
Benefits paid from the pension fund | (17,706) | (541,989) | (690,830) | (308,471)
Benefits paid from the Group | (9,669) | (295,953) | (96,575) | (36,033)
Business combinations | 82,418 | 2,522,805 |  |
Liabilities assumed in a business combination |  |  |  | 535
Exchange differences on foreign plans | (851) | (26,036) | 198,790 | (99,404)
Ending balance | 336,398 | 10,297,139 | 7,910,638 | 8,389,884
Fair value of plan assets [member] |  |  |  |
Disclosure of net defined benefit liability (asset) [line items] |  |  |  |
Beginning balance | (141,829) | (4,341,373) | (4,417,367) | (3,973,729)
Net interest expense (income) | (4,009) | (122,709) | (103,741) | (109,080)
Recognized in profit or loss | (4,009) | (122,709) | (103,741) | (109,080)
Return on plan assets (excluding amounts included in net interest) | (542) | (16,589) | 52,124 | 54,549
Recognized in other comprehensive income | (542) | (16,589) | 52,124 | 54,549
Contributions from the employer | (11,899) | (364,237) | (484,790) | (807,232)
Benefits paid from the pension fund | 17,706 | 541,989 | 690,830 | 308,471
Business combinations | (39,547) | (1,210,524) |  |
Liabilities assumed in a business combination |  |  |  | (535)
Exchange differences on foreign plans | 697 | 21,320 | (78,429) | 110,189
Ending balance | $ (179,423) | $ (5,492,123) | $ (4,341,373) | $ (4,417,367)

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- Definition Actuarial gains (losses) arising from changes in other actuarial assumptions. + References No definition available. + Details
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- Definition Actuarial gains losses arising from experience adjustments. + References No definition available. + Details
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- Definition Post-employment benefit expense, defined benefit plans excluding executive managers. + 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 2018-01-01 -Paragraph 141 -Subparagraph c -Clause ii -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=19&date=2018-03-01&anchor=para_141_c_ii&doctype=Standard -URIDate 2018-03-16 + 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 2018-01-01 -Paragraph 141 -Subparagraph c -Clause iii -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=19&date=2018-03-01&anchor=para_141_c_iii&doctype=Standard -URIDate 2018-03-16 + Details
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- Definition The amount of benefits paid or payable for retirement benefit plans. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Name IAS -Number 26 -IssueDate 2018-01-01 -Paragraph 35 -Subparagraph b -Clause v -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=26&date=2018-03-01&anchor=para_35_b_v&doctype=Standard -URIDate 2018-03-16 + 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 2018-01-01 -Paragraph 141 -Subparagraph f -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=19&date=2018-03-01&anchor=para_141_f&doctype=Standard -URIDate 2018-03-16 + 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 2018-01-01 -Paragraph 141 -Subparagraph a -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=19&date=2018-03-01&anchor=para_141_a&doctype=Standard -URIDate 2018-03-16 + 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 2018-01-01 -Paragraph 141 -Subparagraph h -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=19&date=2018-03-01&anchor=para_141_h&doctype=Standard -URIDate 2018-03-16 + Details
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- Definition The increase (decrease) in the net defined benefit liability (asset) resulting from changes in foreign exchange rates. [Refer: Net defined benefit liability (asset)] + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Name IAS -Number 19 -IssueDate 2018-01-01 -Paragraph 141 -Subparagraph e -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=19&date=2018-03-01&anchor=para_141_e&doctype=Standard -URIDate 2018-03-16 + 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 2018-01-01 -Paragraph 141 -Subparagraph b -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=19&date=2018-03-01&anchor=para_141_b&doctype=Standard -URIDate 2018-03-16 + 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 2018-01-01 -Paragraph 140 -Subparagraph a -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=19&date=2018-03-01&anchor=para_140_a&doctype=Standard -URIDate 2018-03-16 + 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 2018-01-01 -Paragraph 91 -Subparagraph b -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=1&date=2018-03-01&anchor=para_91_b&doctype=Standard -URIDate 2018-03-16 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Name IAS -Number 1 -IssueDate 2018-01-01 -Paragraph 7 -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=1&date=2018-03-01&anchor=para_7&doctype=Standard -URIDate 2018-03-16 + Details
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- Definition The increase (decrease) in the net defined benefit liability (asset) resulting from past service cost. Past service cost is the change in the present value of the defined benefit obligation for employee service in prior periods, resulting from a plan amendment (the introduction or withdrawal of, or changes to, a defined benefit plan) or a curtailment (a significant reduction by the entity in the number of employees covered by a plan). [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 2018-01-01 -Paragraph 141 -Subparagraph d -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=19&date=2018-03-01&anchor=para_141_d&doctype=Standard -URIDate 2018-03-16 + Details
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- Definition The decrease (increase) in the net defined benefit liability (asset) resulting from payments from the plan. [Refer: Net defined benefit liability (asset)] + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Name IAS -Number 19 -IssueDate 2018-01-01 -Paragraph 141 -Subparagraph g -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=19&date=2018-03-01&anchor=para_141_g&doctype=Standard -URIDate 2018-03-16 + 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 2018-01-01 -Paragraph 141 -Subparagraph c -Clause i -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=19&date=2018-03-01&anchor=para_141_c_i&doctype=Standard -URIDate 2018-03-16 + Details
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