Contract Receivables - Additional Information (Details)	12 Months Ended
Dec. 31, 2017
Receivables [Abstract]
Majority of unbilled receivables will be substantially billed and collected	1 year

Represents the the period the majority of unbilled receivables that will be substantially billed and collected.

No definition available.

Name:	icfi_MajorityOfUnbilledReceivablesWillBeSubstantiallyBilledAndCollected1
Namespace Prefix:	icfi_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration