STOCK-BASED COMPENSATION - Valuation and Expenses Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Valuation and Expenses Information
Total stock-based compensation expense	$ 15,317	$ 12,774	$ 6,852
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%	67.00%	70.00%
Expected stock price volatility, maximum (as a percent)	77.00%	69.00%	72.00%
Expected option term	6 years	6 years	6 years
Risk-free interest rate, minimum (as a percent)	1.40%	1.80%	1.90%
Risk-free interest rate, maximum (as a percent)	2.30%	2.20%	2.20%
Expected dividend yield (as a percent)	0.00%	0.00%	0.00%
Research and development
Valuation and Expenses Information
Total stock-based compensation expense	$ 7,821	$ 6,186	$ 3,459
General and administrative
Valuation and Expenses Information
Total stock-based compensation expense	$ 7,496	$ 6,588	$ 3,393