STOCK-BASED COMPENSATION - VALUATION AND EXPENSES INFORMATION (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Valuation and Expenses Information
Total stock-based compensation expense (in dollars)	$ 3,940	$ 2,281
Stock options
Assumptions used for fair values of employee and director stock options granted
Expected stock price volatility (as a percent)	70.00%	69.00%
Expected option term	6 years	6 years
Risk-free interest rate (as a percent)	1.60%	1.80%
Expected dividend yield (as a percent)	0.00%	0.00%
Research and development
Valuation and Expenses Information
Total stock-based compensation expense (in dollars)	$ 1,815	$ 1,269
General and administrative
Valuation and Expenses Information
Total stock-based compensation expense (in dollars)	$ 2,125	$ 1,012