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v3.19.1

Equity - Summary of Treasury Shares (Detail) - shares shares in Thousands | 12 Months Ended
--- | ---
Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Beginning balance | 265,883 | 265,883 | 265,883
Addition | 1,852 | 0 | 0
Decrease | (194,794) | 0 | 0
Ending balance | 72,941 | 265,883 | 265,883
Shares held by subsidiaries [member] |  |  |
Beginning balance | 145,883 | 145,883 | 145,883
Addition |  | 0 | 0
Decrease | (72,942) | 0 | 0
Ending balance | 72,941 | 145,883 | 145,883
Shares reserved for bonds conversion [member] |  |  |
Beginning balance | 120,000 | 120,000 | 120,000
Addition |  | 0 | 0
Decrease | (120,000) | 0 | 0
Ending balance |  | 120,000 | 120,000
Equity Shares Held by Controlled Trust [member] |  |  |
Addition | 1,852 |  |
Decrease | (1,852) |  |

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- Definition Decrease in number of treasury shares disposed of during the period. + References No definition available. + Details
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- Definition Number Of Treasury Shares Outstanding + References No definition available. + Details
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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
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Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | duration

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- Details
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- Details
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Period Type: |

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- Details
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