Stock-Based Compensation - Stock Option Activity (Details) - Employee Stock Option - $ / shares	3 Months Ended	12 Months Ended
	Mar. 31, 2026	Dec. 31, 2025
Shares
Options outstanding at beginning of the period	9,134,278
Granted	78,100
Exercised	(7,442)
Canceled	(133,333)
Options outstanding at the end of the period	9,071,603	9,134,278
Options vested and expected to vest at the end of the period	8,974,496
Options exercisable at the end of the period	5,488,328
Shares available for grant under the Celldex Therapeutics, Inc. 2021 Omnibus Equity Incentive Plan (as amended, effective as of June 5, 2025) at March 31, 2026	2,680,069
Weighted Average Exercise Price Per Share
Options outstanding at beginning of the period	$ 26.49
Granted	27.98
Exercised	14.65
Canceled	33.17
Options outstanding at the end of the period	26.41	$ 26.49
Options vested and expected to Vest at the end of the period	26.41
Options exercisable at the end of the period	$ 26.49
Weighted Average Remaining Contractual Term (In Years)
Options outstanding at the beginning of the period	6 years 10 months 24 days	7 years 1 month 6 days
Options vested and expected to vest at the end of the period	6 years 10 months 24 days
Options exercisable at the end of the period	5 years 9 months 18 days