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Retirement Benefit Plans - Summary of Defined Benefit Plans Amounts Included in Consolidated Balance Sheets (Detail) $ in Thousands, $ in Thousands	Dec. 31, 2020 TWD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2019 TWD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 TWD ($)	Dec. 31, 2017 TWD ($)
Disclosure of net defined benefit liability (asset) [abstract]
Present value of the defined benefit obligation	$ 12,159,145	$ 433,018	$ 10,668,574
Fair value of the plan assets	(5,962,305)	(212,333)	(5,742,178)
Present value of unfunded defined benefit obligation	6,196,840	220,685	4,926,396	$ 175,442	$ 4,805,016	$ 3,569,265
Recorded under other payables	(102,367)	(3,646)	(19,014)
Recorded under other current assets	26,306	937	11,910
Net defined benefit liability	$ 6,120,779	$ 217,976	$ 4,919,292

X

- Definition

Defined benefit plan recorded under others payables.

+ References

No definition available.

+ Details

Name:	asx_DefinedBenefitPlanRecordedUnderOthersPayables
Namespace Prefix:	asx_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

X

- Definition

Net defined benefit liability.

+ References

No definition available.

+ Details

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Namespace Prefix:	asx_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

X

- Definition

Recorded under other current assets.

+ References

No definition available.

+ Details

Name:	asx_RecordedUnderOtherCurrentAssets
Namespace Prefix:	asx_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

X

- Definition

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]

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Name IAS
-Number 19
-IssueDate 2020-01-01
-Paragraph 57
-Subparagraph a
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=19&code=ifrs-tx-2020-en-r&anchor=para_57_a&doctype=Standard
-URIDate 2020-01-01

+ Details

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

X

- References

No definition available.

+ Details

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Namespace Prefix:	ifrs-full_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- 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
-URIDate 2020-01-01

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Balance Type:	credit
Period Type:	instant

X

- 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]]

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Name IAS
-Number 19
-IssueDate 2020-01-01
-Paragraph 57
-Subparagraph a
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=19&code=ifrs-tx-2020-en-r&anchor=para_57_a&doctype=Standard
-URIDate 2020-01-01

+ Details

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