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Income Tax - Additional Information (Detail) - TWD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income tax [Line Items]
Applicable tax rate		17.00%	17.00%	17.00%
Creditable ratios for distribution of earnings			20.48%
Unappropriated earnings [Member]
Income tax [Line Items]
Applicable tax rate		10.00%
Scenario Plan
Income tax [Line Items]
Applicable tax rate	20.00%
Increase in deferred tax assets	$ 480
Increase in deferred tax liabilities	$ 236
Scenario Plan | Unappropriated earnings [Member]
Income tax [Line Items]
Applicable tax rate	5.00%
China [Member]
Income tax [Line Items]
Applicable tax rate		25.00%	25.00%	25.00%

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- Definition

Income tax.

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- Definition

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- Definition

The applicable income tax rate.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 12
-IssueDate 2016-01-13
-Paragraph 81
-Subparagraph c
-Clause ii
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=12&date=2016-03-30&anchor=para_81_c_ii&doctype=Standard
-URIDate 2016-03-31

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