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Retirement Benefit Plans - Pension Costs Recognized in Profit and Loss for Defined Benefit Plans (Detail) - TWD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Disclosure of defined benefit plans [Line Items] |  |  |
Defined benefit plans pension costs recognized in profit and loss | $ 2,834 | $ 2,822 | $ 2,914
Operating costs [Member] |  |  |
Disclosure of defined benefit plans [Line Items] |  |  |
Defined benefit plans pension costs recognized in profit and loss | 1,734 | 1,732 | 1,794
Marketing expenses [Member] |  |  |
Disclosure of defined benefit plans [Line Items] |  |  |
Defined benefit plans pension costs recognized in profit and loss | 847 | 838 | 856
General and administrative expenses [Member] |  |  |
Disclosure of defined benefit plans [Line Items] |  |  |
Defined benefit plans pension costs recognized in profit and loss | 156 | 155 | 162
Research and development expenses [Member] |  |  |
Disclosure of defined benefit plans [Line Items] |  |  |
Defined benefit plans pension costs recognized in profit and loss | $ 97 | $ 97 | $ 102

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- Definition The amount of post-employment benefit expense relating to defined benefit plans. [Refer: Defined benefit plans [member]] + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Name IAS -Number 19 -IssueDate 2016-01-13 -Paragraph 5 -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=19&date=2016-03-30&anchor=para_5&doctype=Standard -URIDate 2016-03-31 + Details
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