Held-to-maturity Financial Assets Current - Summary of Held-to-maturity Financial Assets Current and Related Information (Detail) - TWD ($)	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016
Disclosure of financial assets [Line Items]
Held-to-maturity financial assets current	$ 0	$ 2,140,000,000
Corporate bonds [Member]
Disclosure of financial assets [Line Items]
Held-to-maturity financial assets current		1,990,000,000
Par value		$ 1,990,000,000
Average remaining maturity life	—	0.34 year
Corporate bonds [Member] \| Bottom of range [Member]
Disclosure of financial assets [Line Items]
Nominal interest rate		1.18%
Effective interest rate		1.20%
Corporate bonds [Member] \| Top of range [Member]
Disclosure of financial assets [Line Items]
Nominal interest rate		1.35%
Effective interest rate		1.35%
Bank debentures [Member]
Disclosure of financial assets [Line Items]
Held-to-maturity financial assets current		$ 150,000,000
Par value		$ 150,000,000
Nominal interest rate		1.25%
Effective interest rate		1.25%
Average remaining maturity life	—	0.41 year

The amount of non-derivative financial assets with fixed or determinable payments and fixed maturity that an entity has the positive intention and ability to hold to maturity other than: (a) those that the entity upon initial recognition designates as at fair value through profit or loss; (b) those that the entity designates as available for sale; and (c) those that meet the definition of loans and receivables. An entity shall not classify any financial assets as held to maturity if the entity has, during the current financial year or during the two preceding financial years, sold or reclassified more than an insignificant amount of held-to-maturity investments before maturity (more than insignificant in relation to the total amount of held-to-maturity investments) other than sales or reclassifications that: (i) are so close to maturity or the financial asset’s call date (for example, less than three months before maturity) that changes in the market rate of interest would not have a significant effect on the financial asset’s fair value; (ii) occur after the entity has collected substantially all of the financial asset’s original principal through scheduled payments or prepayments; or (iii) are attributable to an isolated event that is beyond the entity’s control, is non-recurring and could not have been reasonably anticipated by the entity. [Refer: Derivative financial assets; Held-to-maturity investments; Prepayments]

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Note Expiry date 2018-01-01
-Name IFRS
-Number 7
-IssueDate 2016-01-13
-Paragraph 8
-Subparagraph b
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IFRS&num=7&date=2016-03-30&anchor=para_8_b&doctype=Standard
-URIDate 2016-03-31

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