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Financial Risk Management and Fair Values of Financial Instruments - Summary of Loss Rate Methodology (Detail) - TWD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2019 | Dec. 31, 2018
Contract assets [member] |  |
Disclosure of detailed information about financial instruments [Line Items] |  |
Expected loss rate | 0.03% | 0.045%
Total carrying amount | $ 377,983 | $ 299,970
Loss allowance | $ (114) | $ (135)
Accounts and notes receivable including related parties [member] |  |
Disclosure of detailed information about financial instruments [Line Items] |  |
Expected loss rate | 0.03% | 0.045%
Total carrying amount | $ 4,456,065 | $ 4,749,569
Loss allowance | $ (1,351) | $ (2,141)
Other receivable including related parties [member] |  |
Disclosure of detailed information about financial instruments [Line Items] |  |
Expected loss rate | 0.03% | 0.045%
Total carrying amount | $ 92,642 | $ 66,181
Loss allowance | $ (18) | $ (13)

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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
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- Definition The rate of expected credit losses, calculated as percentage of the gross carrying amount. Expected credit losses are the weighted average of credit losses with the respective risks of a default occurring as the weights. + References Reference 1: http://www.xbrl.org/2003/role/exampleRef -Name IFRS -Number 7 -IssueDate 2019-01-01 -Paragraph IG20D -URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IFRS&num=7&code=ifrs-tx-2019-en-r&anchor=para_IG20D&doctype=Implementation%20Guidance -URIDate 2019-03-27 Reference 2: http://www.xbrl.org/2003/role/exampleRef -Name IFRS -Number 7 -IssueDate 2019-01-01 -Paragraph 35N -URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IFRS&num=7&code=ifrs-tx-2019-en-r&anchor=para_35N&doctype=Standard -URIDate 2019-03-27 + Details
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Balance Type: | na
Period Type: | instant

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- Definition Carrying value of net assets. + References No definition available. + Details
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Namespace Prefix: | imos_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition Loss allowance + References No definition available. + Details
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Namespace Prefix: | imos_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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

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

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