STOCK-BASED COMPENSATION - SUMMARY OF PLANS (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
2004 ESPP Plan
Stock-Based Compensation
Common stock reserved for issuance (in shares)	200,000
Purchase price as a percentage of the lower of the fair market value of the common stock at either the beginning of the offering period or the applicable exercise date	85.00%
Shares issued under the plan	15,755	10,798
Shares
Shares Available for Grant (in shares)	137,571
Celldex Research 2005 Plan
Shares
Shares Available for Grant (in shares)	0
2008 Plan
Stock-Based Compensation
Maximum shares of common stock to be issued for grants (in shares)	14,350,000
Terms of options granted to the holders of more than 10% of the voting stock	5 years
Threshold percentage of voting stock	10.00%
Exercise price as a percentage of fair market value for options granted to the holders of more than 10% of the voting stock	110.00%
2008 Plan | Maximum
Stock-Based Compensation
Vesting period	4 years
Terms of options	10 years
2008 Plan | Stock options
Shares
Options Outstanding at beginning of the period (in shares)	7,015,350
Granted (in shares)	1,865,250
Exercised (in shares)	(700,561)
Canceled (in shares)	(69,800)
Options Outstanding at the end of the period (in shares)	8,110,239	7,015,350
Options Vested and Expected to Vest at the end of the period (in shares)	8,065,256
Options Exercisable at the end of the period (in shares)	4,705,304
Shares Available for Grant (in shares)	5,790,709
Weighted Average Exercise Price Per Share
Options Outstanding at beginning of the period (in dollars per share)	$ 9.34
Granted (in dollars per share)	24.75
Exercised (in dollars per share)	5.81
Canceled (in dollars per share)	15.79
Options Outstanding at the end of the period (in dollars per share)	13.13	$ 9.34
Options Vested and Expected to Vest at the end of the period (in dollars per share)	13.08
Options Exercisable at the end of the period (in dollars per share)	$ 8.53
Weighted Average Remaining Contractual Term
Options Outstanding at the beginning of the period	6 years 8 months 12 days	6 years 7 months 6 days
Options Outstanding at the end of the period	6 years 8 months 12 days	6 years 7 months 6 days
Options Vested and Expected to Vest at the end of the period	6 years 8 months 12 days
Options Exercisable at the end of the period	5 years 3 months 18 days
Additional information
Total intrinsic value of stock options exercised (in dollars)	$ 14.4	$ 2.7	$ 10.0
Weighted average grant-date fair value (in dollars per share)	$ 15.25	$ 8.82	$ 10.51
Fair value of stock options vested (in dollars)	$ 10.0	$ 5.7	$ 2.4
Aggregate intrinsic value of stock options outstanding (in dollars)	38.6
Aggregate intrinsic value of stock options vested and expected to vest (in dollars)	38.6
Total compensation cost not yet recognized (in dollars)	$ 36.2
Weighted average period over which compensation cost is expected to be recognized as expense	2 years 7 months 6 days
2008 Plan | Restricted stock
Shares
Outstanding and unvested at the beginning of the period (in shares)	7,000
Granted (in shares)	39,000
Vested (in shares)	(26,500)
Outstanding and unvested at the end of the period (in shares)	19,500	7,000
Weighted Average Grant Date Fair Value (per share)
Outstanding and unvested at the beginning of the period (in dollars per share)	$ 14.78
Granted (in dollars per share)	25.41
Vested (in dollars per share)	22.60
Outstanding and unvested at the end of the period (in dollars per share)	$ 25.41	$ 14.78