| Cover | Feb. 28, 2022 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Amendment Flag | false |
| Document Period End Date | Feb. 28, 2022 |
| Entity File Number | 000-15006 |
| Entity Registrant Name | Celldex Therapeutics, Inc. |
| Entity Central Index Key | 0000744218 |
| Entity Tax Identification Number | 13-3191702 |
| Entity Incorporation, State or Country Code | DE |
| Entity Address, Address Line One | Perryville III Building, 53 Frontage Road, Suite 220 |
| Entity Address, City or Town | Hampton |
| Entity Address, State or Province | NJ |
| Entity Address, Postal Zip Code | 08827 |
| City Area Code | 908 |
| Local Phone Number | 200-7500 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Title of 12(b) Security | Common Stock, par value $.001 |
| Trading Symbol | CLDX |
| Security Exchange Name | NASDAQ |
| Entity Emerging Growth Company | false |
