Stock-Based Compensation - Stock Option Activity (Details) - Employee Stock Option - $ / shares	3 Months Ended	12 Months Ended
	Mar. 31, 2025	Dec. 31, 2024
Shares
Options outstanding at beginning of the period	7,540,109
Granted	40,400
Exercised	(380)
Canceled	(81,381)
Options outstanding at the end of the period	7,498,748	7,540,109
Options vested and expected to vest at the end of the period	7,400,964
Options exercisable at the end of the period	4,233,279
Shares available for grant under the Celldex Therapeutics, Inc. 2021 Omnibus Equity Incentive Plan (as amended, effective as of June 13, 2024) at March 31, 2025	2,413,314
Weighted Average Exercise Price Per Share
Options outstanding at beginning of the period	$ 31.47
Granted	22.04
Exercised	8.38
Canceled	39.24
Options outstanding at the end of the period	31.34	$ 31.47
Options vested and expected to Vest at the end of the period	31.29
Options exercisable at the end of the period	$ 29.14
Weighted Average Remaining Contractual Term (In Years)
Options outstanding at the beginning of the period	7 years 2 months 12 days	7 years 6 months
Options vested and expected to vest at the end of the period	7 years 2 months 12 days
Options exercisable at the end of the period	6 years 1 month 6 days