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Trade Receivables, Net - Additional Information (Detail) - Customers | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016
Disclosure of financial assets [Line Items] |  |
Number of largest customers | 5 | 5
Percentage of accounts receivable from largest customers | 33.00% | 30.00%
Bottom of range [member] |  |
Disclosure of financial assets [Line Items] |  |
Average credit terms | 30 days |
Top of range [member] |  |
Disclosure of financial assets [Line Items] |  |
Average credit terms | 90 days |

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- Definition Average credit terms. + References No definition available. + Details
Name: | asx_AverageCreditTerms
Namespace Prefix: | asx_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

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- Definition Number of largest customers. + References No definition available. + Details
Name: | asx_NumberOfLargestCustomers
Namespace Prefix: | asx_
Data Type: | xbrli:decimalItemType
Balance Type: | na
Period Type: | duration

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- Definition Percentage of accounts receivable from largest customers. + References No definition available. + Details
Name: | asx_PercentageOfAccountsReceivableFromLargestCustomers
Namespace Prefix: | asx_
Data Type: | num:percentItemType
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_DisclosureOfFinancialAssetsLineItems
Namespace Prefix: | ifrs-full_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | ifrs-full_RangeAxis=ifrs-full_BottomOfRangeMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | ifrs-full_RangeAxis=ifrs-full_TopOfRangeMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |