Investment Properties - Key Assumptions and Fair Values of Investment Properties (Detail) $ in Millions, $ in Millions	12 Months Ended
Dec. 31, 2017 TWD ($)	Dec. 31, 2016 TWD ($)	Dec. 31, 2017 USD ($)
Disclosure of detailed information about investment property [Line Items]
Investment properties	$ 8,048	$ 8,115	$ 272
Discount rate	1.04%	1.04%
Bottom of range [Member]
Disclosure of detailed information about investment property [Line Items]
Overall capital interest rate	1.46%	1.46%
Profit margin ratio	12.00%	10.00%
Capitalization rate	0.47%	0.43%
Top of range [Member]
Disclosure of detailed information about investment property [Line Items]
Overall capital interest rate	2.20%	2.20%
Profit margin ratio	20.00%	20.00%
Capitalization rate	1.69%	1.78%
At fair value [Member]
Disclosure of detailed information about investment property [Line Items]
Investment properties	$ 17,728	$ 17,778

Expiry date 2019-01-01: The amount of property (land or a building - or part of a building - or both) held (by the owner or by the lessee under a finance lease) to earn rentals or for capital appreciation or both, rather than for: (a) use in the production or supply of goods or services or for administrative purposes; or (b) sale in the ordinary course of business. Effective 2019-01-01: The amount of property (land or a building - or part of a building - or both) held (by the owner or by the lessee as a right-of-use asset) to earn rentals or for capital appreciation or both, rather than for: (a) use in the production or supply of goods or services or for administrative purposes; or (b) sale in the ordinary course of business.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 40
-IssueDate 2016-01-13
-Paragraph 76
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=40&date=2016-03-30&anchor=para_76&doctype=Standard
-URIDate 2016-03-31

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 40
-IssueDate 2016-01-13
-Paragraph 79
-Subparagraph d
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=40&date=2016-03-30&anchor=para_79_d&doctype=Standard
-URIDate 2016-03-31

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 1
-IssueDate 2016-01-13
-Paragraph 54
-Subparagraph b
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=1&date=2016-03-30&anchor=para_54_b&doctype=Standard
-URIDate 2016-03-31

Name:	ifrs-full_InvestmentProperty
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant