Stock-Based Compensation - Stock Option Activity (Details) - Employee Stock Option - $ / shares	9 Months Ended	12 Months Ended
	Sep. 30, 2024	Dec. 31, 2023
Shares
Options outstanding at beginning of the period	6,378,924
Granted	1,907,820
Exercised	(649,672)
Canceled	(109,968)
Options outstanding at the end of the period	7,527,104	6,378,924
Options vested and expected to vest at the end of the period	7,381,038
Options exercisable at the end of the period	3,746,377
Shares available for grant under the Celldex Therapeutics, Inc. 2021 Omnibus Equity Incentive Plan (as amended, effective as of June 13, 2024) at September 30, 2024	2,415,651
Weighted Average Exercise Price Per Share
Options outstanding at beginning of the period	$ 29.69
Granted	36.42
Exercised	12.99
Canceled	120.83
Options outstanding at the end of the period	31.51	$ 29.69
Options vested and expected to Vest at the end of the period	31.44
Options exercisable at the end of the period	$ 29.19
Weighted Average Remaining Contractual Term (In Years)
Options outstanding at the end of the period	7 years 8 months 12 days	7 years 6 months
Options vested and expected to vest at the end of the period	7 years 8 months 12 days
Options exercisable at the end of the period	6 years 4 months 24 days