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Retirement Benefit Plans - Effect on Present Value of Defined Benefit Obligation Due to a Reasonably Possible Change of the Significant Actuarial Assumptions (Detail) - TWD ($) $ in Millions | Dec. 31, 2017 | Dec. 31, 2016
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Discount rates [Member] |  |
Disclosure of sensitivity analysis for actuarial assumptions [Line Items] |  |
0.5% increase | $ (1,232) | $ (1,219)
0.5% decrease | 1,310 | 1,298
Expected rates of salary increases [Member] |  |
Disclosure of sensitivity analysis for actuarial assumptions [Line Items] |  |
0.5% increase | 1,398 | 1,379
0.5% decrease | $ (1,326) | $ (1,306)

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- Definition The increase (decrease) in a defined benefit obligation that would have been caused by a decrease in a significant actuarial assumption that was reasonably possible at the end of the reporting period. [Refer: Actuarial assumptions [member]] + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Name IAS -Number 19 -IssueDate 2016-01-13 -Paragraph 145 -Subparagraph a -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=19&date=2016-03-30&anchor=para_145_a&doctype=Standard -URIDate 2016-03-31 + Details
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- Definition The increase (decrease) in a defined benefit obligation that would have been caused by an increase in a significant actuarial assumption that was reasonably possible at the end of the reporting period. [Refer: Actuarial assumptions [member]] + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Name IAS -Number 19 -IssueDate 2016-01-13 -Paragraph 145 -Subparagraph a -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=19&date=2016-03-30&anchor=para_145_a&doctype=Standard -URIDate 2016-03-31 + Details
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