STOCK-BASED COMPENSATION (Details) (USD $) In Millions, except Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Stock options
Stock-Based Compensation
Shares available for issuance	675,159
Shares
Options Outstanding at beginning of the period (in shares)	5,770,544
Granted (in shares)	1,456,800
Exercised (in shares)	(169,977)
Canceled (in shares)	(42,017)
Options Outstanding at the end of the period (in shares)	7,015,350	5,770,544
Options Vested and Expected to Vest at the end of the period (in shares)	6,963,570
Options Exercisable at the end of the period (in shares)	4,090,112
Shares Available for Grant under the 2008 Plan (in shares)	675,159
Weighted Average Exercise Price Per Share
Options Outstanding at beginning of the period (in dollars per share)	$ 8.17
Granted (in dollars per share)	$ 13.69
Exercised (in dollars per share)	$ 6.00
Canceled (in dollars per share)	$ 13.10
Options Outstanding at the end of the period (in dollars per share)	$ 9.34	$ 8.17
Options Vested and Expected to Vest at the end of the period (in dollars per share)	$ 9.31
Options Exercisable at the end of the period (in dollars per share)	$ 7.06
Weighted Average Remaining Contractual Term
Options Outstanding at beginning of the period	6 years 7 months 6 days	7 years
Options Outstanding at the end of the period	6 years 7 months 6 days	7 years
Options Vested and Expected to Vest at the end of the period	6 years 7 months 6 days
ptions Exercisable at the end of the period	5 years 1 month 6 days
Additional information
Total intrinsic value of stock options exercised (in dollars)	$ 2.7	$ 10.0	$ 0.2
Weighted average grant-date fair value (in dollars per share)	$ 8.82	$ 10.51	$ 3.67
Fair value of stock options vested (in dollars)	5.7	2.4	2.1
Aggregate intrinsic value of stock options outstanding (in dollars)	62.8
Aggregate intrinsic value of stock options vested and expected to vest (in dollars)	62.5
Total compensation cost not yet recognized (in dollars)	$ 20.3
Weighted average period over which compensation cost is expected to be recognized as expense	2 years 7 months 6 days
Restricted stock
Shares
Outstanding and unvested at the beginning of the period (in shares)	6,000
Granted (in shares)	14,000
Vested (in shares)	(12,000)
Canceled (in shares)	(1,000)
Outstanding and unvested at the end of the period (in shares)	7,000
Weighted Average Grant Date Fair Value (per share)
Outstanding and unvested at the beginning of the period (in dollars per share)	$ 15.20
Granted (in dollars per share)	$ 14.12
Vested (in dollars per share)	$ 14.33
Outstanding and unvested at the end of the period (in dollars per share)	$ 14.78
Canceled (in dollars per share)	$ 13.45
Celldex Research 2005 Plan and CuraGen 2007 Plan
Stock-Based Compensation
Shares available for issuance	0
Shares
Shares Available for Grant under the 2008 Plan (in shares)	0
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	10,798	11,823
Shares available for issuance	153,326
Shares
Shares Available for Grant under the 2008 Plan (in shares)	153,326
2008 Plan
Stock-Based Compensation
Maximum shares of common stock to be issued for grants (in shares)	7,400,000
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