STOCK-BASED COMPENSATION - VALUATION AND EXPENSES INFORMATION (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Valuation and Expenses Information
Total stock-based compensation expense (in dollars)	$ 12,774	$ 6,852	$ 4,548
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)	67.00%	70.00%	71.00%
Expected stock price volatility, maximum (as a percent)	69.00%	72.00%	72.00%
Expected option term	6 years	6 years	6 years
Risk-free interest rate, minimum (as a percent)	1.80%	1.90%	1.20%
Risk-free interest rate, maximum (as a percent)	2.20%	2.20%	2.10%
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)	$ 6,186	$ 3,459	$ 2,640
General and administrative
Valuation and Expenses Information
Total stock-based compensation expense (in dollars)	$ 6,588	$ 3,393	$ 1,908