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Document And Entity Information - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2023
Feb. 23, 2024
Jun. 30, 2023
Cover [Abstract]      
Entity Registrant Name ICF INTERNATIONAL, INC.    
Entity Central Index Key 0001362004    
Current Fiscal Year End Date --12-31    
Entity Filer Category Large Accelerated Filer    
Entity Small Business false    
Entity Emerging Growth Company false    
Entity Shell Company false    
Document Annual Report true    
Document Transition Report false    
Entity Current Reporting Status Yes    
Entity Voluntary Filers No    
Entity Interactive Data Current Yes    
Entity Well-known Seasoned Issuer No    
Entity Common Stock, Shares Outstanding (in shares)   18,715,376  
Entity Public Float     $ 2,309
Document Type 10-K/A    
Document Period End Date Dec. 31, 2023    
Document Fiscal Year Focus 2023    
Document Fiscal Period Focus FY    
Amendment Flag true    
Amendment Description This Amendment No. 1 to Form 10-K (“Form 10-K/A”) amends the Annual Report on Form 10-K of ICF International, Inc. (the “Company”) for the year ended December 31, 2023, previously filed with the Securities and Exchange Commission (the “SEC”) on February 28, 2024 (“Form 10-K”).The Form 10-K/A is being filed to replace the consent of Grant Thornton LLP (the “Consent”), filed as Exhibit 23.1 to the Form 10-K, with an updated version of the Consent as Exhibit 23.1 to the Form 10-K/A to include the reference to Form S-8 File No. 333-272833, which was inadvertently omitted in the February 28, 2024 filing. Pursuant to Rule 12b-15 under the Security Exchange Act of 1934, as amended (the “Exchange Act”), the Form 10-K/A also contains new certifications pursuant to Section 302 of the Sarbanes-Oxley Act of 2002, which are attached hereto. The Form 10-K/A neither includes financial statements nor amends any disclosures with respect to Items 307 and 308 of Regulation S-K under the Exchange Act, consequently paragraphs 3, 4, and 5 of the certifications have been omitted.Except as set forth in the Form 10-K/A, no other changes have been made to the Form 10-K. Additionally, the Form 10-K has not been amended or updated to reflect events occurring after February 28, 2024, except as specifically set forth in the Form 10-K/A.    
Entity File Number 001-33045    
Entity Tax Identification Number 22-3661438    
ICFR Auditor Attestation Flag true    
Document Financial Statement Error Correction [Flag] false    
Entity Incorporation State Country Code DE    
Entity Address, Address Line One 1902 Reston Metro Plaza    
Entity Address, City or Town Reston    
Entity Address, State or Province VA    
Entity Address, Postal Zip Code 20190    
City Area Code 703    
Local Phone Number 934-3000    
Title of each class Common Stock, $0.001 par value    
Trading Symbol ICFI    
Security Exchange Name NASDAQ    
Documents Incorporated by Reference

List hereunder the following documents if incorporated by reference and the Part of the Form 10-K (e.g., Part I, Part II, etc.) into which the document is incorporated: (1) Any annual report to security holders; (2) Any proxy or information statement; and (3) Any prospectus filed pursuant to Rule 424(b) or (c) under the Securities Act of 1933. The listed documents should be clearly described for identification purposes (e.g., annual report to security holders for fiscal year ended December 24, 1980).

Part III incorporates information by reference from the Proxy Statement for the 2024 Annual Meeting of Stockholders expected to be held in June 2024.