Retirement Benefit Plans - Amounts Included in Consolidated Balance Sheets from Obligation in Respect of Defined Benefit Plans (Detail) $ in Millions, $ in Millions	Dec. 31, 2020 TWD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2019 TWD ($)	Dec. 31, 2018 TWD ($)	Dec. 31, 2017 TWD ($)
Disclosure Of Defined Benefit Plans [Abstract]
Present value of funded defined benefit obligations	$ 39,536		$ 41,197
Fair value of plan assets	(39,494)		(39,820)
Funded status - deficit	42		1,377
Net defined benefit liabilities	3,415	$ 122	3,505
Net defined benefit assets	(3,373)	$ (120)	(2,128)
Defined benefit obligation	$ 42		$ 1,377	$ 2,370	$ 2,691

The present value, without deducting any plan assets, of expected future payments required to settle the obligation resulting from employee service in the current and prior periods. [Refer: Plan assets, at fair value]

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
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-Number 19
-IssueDate 2020-01-01
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-Subparagraph a
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-URIDate 2020-01-01

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- References

No definition available.

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- Definition

The amount of deficit or surplus in a defined benefit plan, adjusted for any effect of limiting a net defined benefit asset to the asset ceiling. [Refer: Defined benefit plans [member]]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 19
-IssueDate 2020-01-01
-Paragraph 140
-Subparagraph a
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=19&code=ifrs-tx-2020-en-r&anchor=para_140_a&doctype=Standard
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- Definition

The amount of non-current net defined benefit asset. [Refer: Net defined benefit asset]

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Name IAS
-Number 1
-IssueDate 2020-01-01
-Paragraph 55
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- Definition

The amount of non-current net defined benefit liability. [Refer: Net defined benefit liability]

+ References

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- Definition

The fair value of defined benefit plan assets. Plan assets comprise assets held by a long-term employee benefit fund and qualifying insurance policies. [Refer: At fair value [member]]

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- Definition

The fair value of any plan assets, less the present value of the defined benefit obligation. [Refer: Plan assets [member]]

+ References

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