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v3.20.1

Financial Statements Schedule: Valuation and Qualifying Accounts (Detail) - TWD ($) $ in Thousands | 12 Months Ended
--- | ---
Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017
Allowance for Impairment of property, plant and equipment [member] |  |  |
Disclosure of details of valuation and qualifying accounts [Line Items] |  |  |
January 1 | $ 317,593 | $ 320,455 | $ 359,270
Additions charged to expense | 9,938 |  | 956
Deduction/Write-offs | (134,191) | (2,862) | (39,771)
December 31 | 193,340 | 317,593 | 320,455
Allowance for Impairment of Obsolescence and Decline in Market Value of Inventories [Member] |  |  |
Disclosure of details of valuation and qualifying accounts [Line Items] |  |  |
January 1 | 36,157 | 53,714 | 154,841
Additions charged to expense | 27,341 |  |
Deduction/Write-offs |  | (17,557) | (101,127)
December 31 | $ 63,498 | $ 36,157 | $ 53,714

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- Definition Valuation allowances and reserves. + References No definition available. + Details
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Namespace Prefix: | imos_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Valuation allowances and reserves charged to expense. + 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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- Definition Valuation allowances and reserves deductions. + References No definition available. + Details
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Namespace Prefix: | imos_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
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Namespace Prefix: | srt_
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: |

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- Details
Name: | imos_ValuationAllowancesAndReservesTypeAxis=imos_AllowanceForImpairmentOfObsolescenceAndDeclineInMarketValueOfInventoriesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |