STOCK-BASED COMPENSATION (Details 2) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Valuation and Expenses Information
Total stock-based compensation expense (in dollars)	$ 6,852	$ 4,548	$ 2,298
Assumptions used for fair values of employee and director stock options granted
Expected dividend payable in foreseeable future	0
Stock options
Assumptions used for fair values of employee and director stock options granted
Expected stock price volatility, minimum (as a percent)	70.00%	71.00%	70.00%
Expected stock price volatility, maximum (as a percent)	72.00%	72.00%	72.00%
Expected option term	6 years	6 years	6 years
Risk-free interest rate, minimum (as a percent)	1.90%	1.20%	0.90%
Risk-free interest rate, maximum (as a percent)	2.20%	2.10%	1.40%
Expected dividend yield (as a percent)	0.00%	0.00%	0.00%
Research and development
Valuation and Expenses Information
Total stock-based compensation expense (in dollars)	3,459	2,640	1,415
General and administrative
Valuation and Expenses Information
Total stock-based compensation expense (in dollars)	$ 3,393	$ 1,908	$ 883