Revenue Recognition - Additional Information (Detail) $ in Thousands	9 Months Ended
Sep. 30, 2019 USD ($)
Disaggregation Of Revenue [Abstract]
Net contract assets (liabilities)	$ 27,775

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