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Contract Receivables, Net - Summary of Reconciliation of Billed Contract Receivables Sold to MUF Including Receivables Sold to MUFC And Collected from Customers on Behalf of MUFG And Receivables Not Yet Collected from Customers (Parenthetical) (Details) - USD ($) $ in Millions | 6 Months Ended
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Jun. 30, 2024 | Jun. 30, 2023
Receivables [Abstract] |  |
Proceeds from sale of billed contract receivables | $ 13.4 | $ 23.8

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- Definition Proceeds from sale of billed contract receivables. + References No definition available. + Details
Name: | icfi_ProceedsFromSaleOfBilledContractReceivables
Namespace Prefix: | icfi_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- References No definition available. + Details
Name: | us-gaap_ReceivablesAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration