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Equity - Summary of Treasury Shares (Detail) - shares | 12 Months Ended
--- | ---
Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Beginning balance | 265,883 | 265,883 | 145,883
Addition |  |  | 120,000
Decrease | 0 | 0 | 0
Ending balance | 265,883 | 265,883 | 265,883
Shares held by subsidiaries [member] |  |  |
Beginning balance | 145,883 | 145,883 | 145,883
Decrease | 0 | 0 | 0
Ending balance | 145,883 | 145,883 | 145,883
Shares reserved for bonds conversion [member] |  |  |
Beginning balance | 120,000 | 120,000 |
Addition |  |  | 120,000
Decrease | 0 | 0 | 0
Ending balance | 120,000 | 120,000 | 120,000

X
- Definition Decrease in number of treasury shares disposed of during the period. + References No definition available. + Details
Name: | asx_DecreaseInNumberOfTreasurySharesDisposedOfDuringThePeriod
Namespace Prefix: | asx_
Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | duration

X
- Definition Number Of Treasury Shares Outstanding + References No definition available. + Details
Name: | asx_NumberOfTreasurySharesOutstanding
Namespace Prefix: | asx_
Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | instant

X
- Definition Purchase of treasury shares shares. + References No definition available. + Details
Name: | asx_PurchaseOfTreasurySharesShares
Namespace Prefix: | asx_
Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | duration

X
- Details
Name: | ifrs-full_ComponentsOfEquityAxis=asx_SharesHeldBySubsidiariesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | ifrs-full_ComponentsOfEquityAxis=asx_BondsAndConvertibleBondsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |