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Leases - Summary of Operating Leases (Details) - USD ($) $ in Thousands | Dec. 31, 2021 | Dec. 31, 2020
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Lessee Lease Description [Line Items] |  |
Total before amortization | $ 240,394 |
Amortization of right-of-use assets | (62,977) |
Total operating lease right-of-use assets | 177,417 | $ 127,132
Real Estate Facilities |  |
Lessee Lease Description [Line Items] |  |
Total before amortization | 237,591 |
Office Equipment |  |
Lessee Lease Description [Line Items] |  |
Total before amortization | 2,117 |
Other |  |
Lessee Lease Description [Line Items] |  |
Total before amortization | $ 686 |

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- Definition Operating lease right of use asset amortization. + References No definition available. + Details
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Namespace Prefix: | icfi_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Operating lease right of use asset gross. + References No definition available. + Details
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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: | us-gaap_
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Period Type: | duration

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- Definition Amount of lessee's right to use underlying asset under operating lease. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 842 -SubTopic 20 -Section 45 -Paragraph 1 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=123391704&loc=SL77918627-209977 + Details
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Namespace Prefix: | us-gaap_
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- Details
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- Details
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- Details
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Namespace Prefix: |
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