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Contingencies - Additional Information (Details) - USD ($) $ in Millions | Jun. 10, 2016 | Sep. 30, 2020
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Loss Contingencies [Line Items] |  |
Community development related to claim |  | $ 220.2
OCD vs ICF Emergency |  |
Loss Contingencies [Line Items] |  |
Loss contingency damages sought value | $ 200.8 |
Road Home Contract |  |
Loss Contingencies [Line Items] |  |
Contract term, period | 3 years |
Contract award, value | $ 912.0 |

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- Definition Community development related to the claim. + 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 The amount of the contract award. + References No definition available. + Details
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Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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Balance Type: | na
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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Period Type: | duration

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- Definition The value (monetary amount) of the award the plaintiff seeks in the legal matter. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 450 -SubTopic 20 -Section 50 -Paragraph 9 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=121557415&loc=d3e14557-108349 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 450 -SubTopic 20 -Section 50 -Paragraph 4 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=121557415&loc=d3e14435-108349 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 450 -SubTopic 20 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=121557415&loc=d3e14326-108349 + Details
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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: |