Document And Entity Information - shares	3 Months Ended
	Mar. 31, 2021	Apr. 30, 2021
Cover [Abstract]
Entity Registrant Name	ICF International, Inc.
Entity Central Index Key	0001362004
Trading Symbol	ICFI
Security Exchange Name	NASDAQ
Current Fiscal Year End Date	--12-31
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Common Stock, Shares Outstanding		18,837,808
Document Type	10-Q
Document Period End Date	Mar. 31, 2021
Document Fiscal Year Focus	2021
Document Fiscal Period Focus	Q1
Amendment Flag	false
Entity Current Reporting Status	Yes
Entity Shell Company	false
Entity File Number	001-33045
Entity Tax Identification Number	22-3661438
Entity Incorporation State Country Code	DE
Entity Address, Address Line One	9300 Lee Highway
Entity Address, City or Town	Fairfax
Entity Address, State or Province	VA
Entity Address, Postal Zip Code	22031
City Area Code	703
Local Phone Number	934-3000
Title of each class	Common Stock
Entity Interactive Data Current	Yes
Document Quarterly Report	true
Document Transition Report	false