Significant contingent liabilities and unrecognized contract commitments - Capital Expenditures that are Contracted for, but not Provided for (Detail) - TWD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Disclosure of detailed information about property, plant and equipment [abstract]
Property plant and equipment, Contracted capital expenditures	$ 2,331,041	$ 1,640,712

The amount of contractual commitments for the acquisition of property, plant and equipment. [Refer: Property, plant and equipment]

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 16
-IssueDate 2020-01-01
-Paragraph 74
-Subparagraph c
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=16&code=ifrs-tx-2020-en-r&anchor=para_74_c&doctype=Standard
-URIDate 2020-01-01

Name:	ifrs-full_ContractualCommitmentsForAcquisitionOfPropertyPlantAndEquipment
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant