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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

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- 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

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- 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

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