Investment Properties - Key Assumptions and Fair Values of Investment Properties (Detail) $ in Millions, $ in Millions	12 Months Ended
Dec. 31, 2018 TWD ($)	Dec. 31, 2017 TWD ($)	Dec. 31, 2018 USD ($)
Disclosure of detailed information about investment property [Line Items]
Investment properties	$ 8,287	$ 8,048	$ 271
Discount rate	0.00%	1.04%
Bottom of range [Member]
Disclosure of detailed information about investment property [Line Items]
Overall capital interest rate	1.02%	1.46%
Profit margin ratio	12.00%	12.00%
Capitalization rate	0.79%	0.47%
Top of Range [Member]
Disclosure of detailed information about investment property [Line Items]
Overall capital interest rate	4.04%	2.20%
Profit margin ratio	20.00%	20.00%
Capitalization rate	1.75%	1.69%
At fair value [Member]
Disclosure of detailed information about investment property [Line Items]
Investment properties	$ 18,515	$ 17,728

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 2018-01-01
-Paragraph 76
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=40&date=2018-03-01&anchor=para_76&doctype=Standard
-URIDate 2018-03-16

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 40
-IssueDate 2018-01-01
-Paragraph 79
-Subparagraph d
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=40&date=2018-03-01&anchor=para_79_d&doctype=Standard
-URIDate 2018-03-16

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 1
-IssueDate 2018-01-01
-Paragraph 54
-Subparagraph b
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=1&date=2018-03-01&anchor=para_54_b&doctype=Standard
-URIDate 2018-03-16

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