Stock-Based Compensation - Stock Option Activity (Details) - Employee Stock Option - $ / shares	12 Months Ended
	Dec. 31, 2025	Dec. 31, 2024
Shares
Options outstanding at beginning of the period	7,540,109
Granted	2,154,350
Exercised	(153,328)
Canceled	(406,853)
Options outstanding at the end of the period	9,134,278	7,540,109
Options vested and expected to vest at the end of the period	9,003,451
Options exercisable at the end of the period	5,284,496
Shares available for grant under the Celldex Therapeutics, Inc. 2021 Omnibus Equity Incentive Plan (as amended, effective as of June 5, 2025) at December 31, 2025	2,624,836
Weighted Average Exercise Price Per Share
Options outstanding at beginning of the period	$ 31.47
Granted	19.96
Exercised	13.8
Canceled	89.06
Options outstanding at the end of the period	26.49	$ 31.47
Options vested and expected to Vest at the end of the period	26.5
Options exercisable at the end of the period	$ 26.34
Weighted Average Remaining Contractual Term (In Years)
Options outstanding at the beginning of the period	7 years 1 month 6 days	7 years 6 months
Options vested and expected to vest at the end of the period	7 years 1 month 6 days
Options exercisable at the end of the period	6 years