Contract Receivables, Net - Additional Information (Details) - USD ($) $ in Millions	9 Months Ended
Sep. 30, 2024	Sep. 30, 2023	Dec. 31, 2023
Financing Receivable, Past Due [Line Items]
Sale of billed contract receivables amount	$ 18.9	$ 32.2
Master Receivables Purchase Agreement with MUFG Bank
Financing Receivable, Past Due [Line Items]
Cash collected not yet remitted for receivables sold	$ 2.7		$ 6.9

Cash collected not yet remitted to purchaser of billed receivables.

No definition available.

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Namespace Prefix:	icfi_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- 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

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 14
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479319/326-20-50-14

Reference 2: http://www.xbrl.org/2003/role/exampleRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 55
-Paragraph 80
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479294/326-20-55-80

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

The cash inflow associated with the proceeds from sale and collection of receivables during the period.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 12
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482740/230-10-45-12

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Details

Name:	icfi_ContractReceivablesAxis=icfi_MasterReceivablesPurchaseAgreementWithMufgBankMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: