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Supplementary Cash Flow Information - Partial Cash Paid for Investing and Financing Activities (Parenthetical) (Detail) - TWD ($) $ in Thousands | Dec. 31, 2018 | Dec. 31, 2017
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Disclosure Of Supplementary Cash Flow Information [line items] |  |
Other payables paid in 2018 |  | $ 64,393
Unimos Shanghai [member] |  |
Disclosure Of Supplementary Cash Flow Information [line items] |  |
Other payables paid in 2018 | $ 64,393 |

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- Definition Amounts payable that the entity does not separately disclose in the same statement or note. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Name IAS -Number 1 -IssueDate 2019-01-01 -Paragraph 55 -URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=1&code=ifrs-tx-2019-en-r&anchor=para_55&doctype=Standard -URIDate 2019-03-27 + Details
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Namespace Prefix: | ifrs-full_
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Balance Type: | credit
Period Type: | instant

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- Definition Disclosure of supplementary cash flow information + References No definition available. + Details
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Namespace Prefix: | imos_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |