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v3.20.2

Revenue Recognition - Additional Information (Detail) $ in Thousands | 6 Months Ended
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Jun. 30, 2020 USD ($)
Revenue From Contract With Customer [Abstract] |
Net contract assets (liabilities) | $ 15,518

X
- Definition Change in contract with customer assets liability net current. + References No definition available. + Details
Name: | icfi_ChangeInContractWithCustomerAssetsLiabilitiesNetCurrent
Namespace Prefix: | icfi_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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