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Financial Assets at Amortized Cost - Summary of Financial Assets at Amortized Cost (Detail) - TWD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Financial Instruments At Amortised Cost Category[abstract] |  |  |
Time deposits | $ 206,482 | $ 168,970 |
Restricted bank deposits | 48,319 | 68,450 |
Interest income | $ 2,206 | $ 4,467 | $ 920

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- Definition The amount of revenue arising from interest for financial assets that are measured at amortised cost. [Refer: Interest income; Financial assets at amortised cost] + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Name IFRS -Number 7 -IssueDate 2020-01-01 -Paragraph 20 -Subparagraph b -URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IFRS&num=7&code=ifrs-tx-2020-en-r&anchor=para_20_b&doctype=Standard -URIDate 2020-01-01 + Details
Name: | ifrs-full_InterestIncomeForFinancialAssetsMeasuredAtAmortisedCost
Namespace Prefix: | ifrs-full_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Definition Financial instruments at amortised cost category. + References No definition available. + Details
Name: | imos_FinancialInstrumentsAtAmortisedCostCategoryAbstract
Namespace Prefix: | imos_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Restricted bank deposits non current. + References No definition available. + Details
Name: | imos_RestrictedBankDepositsNonCurrent
Namespace Prefix: | imos_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition Time deposits current. + References No definition available. + Details
Name: | imos_TimeDepositsCurrent
Namespace Prefix: | imos_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant