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Leases - Summary of Operating Leases (Details) - USD ($) $ in Thousands | Dec. 31, 2020 | Dec. 31, 2019
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Lessee Lease Description [Line Items] |  |
Operating lease right of use assets, gross | $ 159,454 |
Amortization of right-of-use assets | (32,322) |
Total operating lease right-of-use assets | 127,132 | $ 133,965
Real Estate Facilities |  |
Lessee Lease Description [Line Items] |  |
Operating lease right of use assets, gross | 157,010 |
Office Equipment |  |
Lessee Lease Description [Line Items] |  |
Operating lease right of use assets, gross | 1,864 |
Other |  |
Lessee Lease Description [Line Items] |  |
Operating lease right of use assets, gross | $ 580 |

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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=121603541&loc=SL77918627-209977 + Details
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- Details
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- Details
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- Details
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