Revenue Recognition - Additional Information (Detail) $ in Thousands	12 Months Ended
Dec. 31, 2019 USD ($)
Revenue From Contract With Customer [Abstract]
Net contract assets (liabilities)	$ 11,730

Change in contract with customer assets liability net current.

No definition available.

Name:	icfi_ChangeInContractWithCustomerAssetsLiabilitiesNetCurrent
Namespace Prefix:	icfi_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration