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Earnings Per Share - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Earnings per share [line items]
Number of shares represented by each ADS	2	2
Before Retrospectively Adjusted [member]
Earnings per share [line items]
Number of shares represented by each ADS		5	5

X

- Definition

Number of ordinary shares per ADS.

+ References

No definition available.

+ Details

Name:	asx_NumberOfOrdinarySharesPerADS
Namespace Prefix:	asx_
Data Type:	xbrli:decimalItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Line items represent concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes of the table.

+ References

No definition available.

+ Details

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

X

- Details

Name:	ifrs-full_RetrospectiveApplicationAndRetrospectiveRestatementAxis=ifrs-full_PreviouslyStatedMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: