v3.8.0.1
Document And Entity Information - shares
9 Months Ended
Sep. 30, 2017
Oct. 27, 2017
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)   18,642,406
Document Type 10-Q  
Document Period End Date Sep. 30, 2017  
Document Fiscal Year Focus 2017  
Document Fiscal Period Focus Q3  
Amendment Flag false