| Document And Entity Information | Oct. 02, 2019 |
| Document Information [Line Items] | |
| Entity Registrant Name | ICF International, Inc. |
| Document Type | 8-K |
| Document Period End Date | Oct. 2, 2019 |
| Entity Incorporation, State or Country Code | DE |
| Entity File Number | 001-33045 |
| Entity Tax Identification Number | 22-3661438 |
| 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 12(b) Security | Common Stock |
| Trading Symbol | ICFI |
| Security Exchange Name | NASDAQ |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Entity Emerging Growth Company | false |
| Amendment Flag | false |
| Entity Central Index Key | 0001362004 |
