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Retirement Benefit Plans - Summary of Sensitivity Analysis (Detail) $ in Millions | 12 Months Ended
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Dec. 31, 2017 TWD ($) yr | Dec. 31, 2016 TWD ($) yr
Disclosure of sensitivity analysis for actuarial assumptions [Line Items] |  |
The expected contributions to the plan for the next year | $ | $ 4,393 | $ 2,724
Bottom of range [Member] |  |
Disclosure of sensitivity analysis for actuarial assumptions [Line Items] |  |
The average duration of the defined benefit obligation | 6 | 7
Top of range [Member] |  |
Disclosure of sensitivity analysis for actuarial assumptions [Line Items] |  |
The average duration of the defined benefit obligation | 13 | 14

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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 2016-01-13 -Paragraph 147 -Subparagraph b -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=19&date=2016-03-30&anchor=para_147_b&doctype=Standard -URIDate 2016-03-31 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Name IAS -Number 19 -IssueDate 2016-01-13 -Paragraph 148 -Subparagraph d -Clause iii -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=19&date=2016-03-30&anchor=para_148_d_iii&doctype=Standard -URIDate 2016-03-31 + 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 2016-01-13 -Paragraph 147 -Subparagraph c -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=19&date=2016-03-30&anchor=para_147_c&doctype=Standard -URIDate 2016-03-31 + Details
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