v3.10.0.1
Document And Entity Information - shares
6 Months Ended
Jun. 30, 2018
Jul. 27, 2018
Document And Entity Information [Abstract]    
Entity Registrant Name ICF International, Inc.  
Entity Central Index Key 0001362004  
Trading Symbol ICFI  
Current Fiscal Year End Date --12-31  
Entity Filer Category Large Accelerated Filer  
Entity Current Reporting Status Yes  
Entity Voluntary Filers No  
Entity Well-known Seasoned Issuer No  
Entity Common Stock, Shares Outstanding (in shares)   18,860,323
Document Type 10-Q  
Document Period End Date Jun. 30, 2018  
Document Fiscal Year Focus 2018  
Document Fiscal Period Focus Q2  
Amendment Flag false