Revenue Recognition - Additional Information (Detail) $ in Thousands	Dec. 31, 2018 USD ($)
Revenue From Contract With Customer [Line Items]
Net contract assets (liabilities)	$ 93,194
ASU 2014-09 \| Increase in Net Contract Assets (Liabilities)
Revenue From Contract With Customer [Line Items]
Net contract assets (liabilities)	$ 8,568

Contract with customer assets (liabilities), net current.

No definition available.

Name:	icfi_ContractWithCustomerAssetsLiabilitiesNetCurrent
Namespace Prefix:	icfi_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant