STOCK-BASED COMPENSATION - Summary of Plans (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
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	59,335	15,755
Shares
Shares Available for Grant (in shares)	78,236
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)	8,110,239
Granted (in shares)	2,464,750
Exercised (in shares)	(48,817)
Canceled (in shares)	(307,462)
Options Outstanding at the end of the period (in shares)	10,218,710	8,110,239
Options Vested and Expected to Vest at the end of the period (in shares)	10,148,728
Options Exercisable at the end of the period (in shares)	6,086,840
Shares Available for Grant (in shares)	3,577,635
Weighted Average Exercise Price Per Share
Options Outstanding at beginning of the period (in dollars per share)	$ 13.13
Granted (in dollars per share)	4.78
Exercised (in dollars per share)	2.90
Canceled (in dollars per share)	14.05
Options Outstanding at the end of the period (in dollars per share)	11.14	$ 13.13
Options Vested and Expected to Vest at the end of the period (in dollars per share)	11.14
Options Exercisable at the end of the period (in dollars per share)	$ 10.86
Weighted Average Remaining Contractual Term (In Years)
Options Outstanding at the end of the period	6 years 6 months	6 years 8 months 12 days
Options Vested and Expected to Vest at the end of the period	6 years 6 months
Options Exercisable at the end of the period	5 years
Additional information
Total intrinsic value of stock options exercised (in dollars)	$ 0.1	$ 14.4	$ 2.7
Weighted average grant-date fair value (in dollars per share)	$ 3.18	$ 15.25	$ 8.82
Fair value of stock options vested (in dollars)	$ 17.0	$ 10.0	$ 5.7
Aggregate intrinsic value of stock options outstanding (in dollars)	0.4
Aggregate intrinsic value of stock options vested and expected to vest (in dollars)	0.4
Total compensation cost not yet recognized (in dollars)	$ 27.4
Weighted average period over which compensation cost is expected to be recognized as expense	2 years 9 months 18 days
2008 Plan | Restricted stock
Shares
Outstanding and unvested at the beginning of the period (in shares)	19,500
Granted (in shares)	60,000
Vested (in shares)	(19,500)
Canceled (in shares)	(10,000)
Outstanding and unvested at the end of the period (in shares)	50,000	19,500
Weighted Average Grant Date Fair Value (per share)
Outstanding and unvested at the beginning of the period (in dollars per share)	$ 25.41
Granted (in dollars per share)	4.72
Vested (in dollars per share)	25.41
Canceled (in dollars per share)	4.72
Outstanding and unvested at the end of the period (in dollars per share)	$ 4.72	$ 25.41