SHARE-BASED COMPENSATION (Tables)

12 Months Ended

Dec. 31, 2018

Disclosure of Compensation Related Costs, Share-based Payments [Abstract]

Schedule of changes to the number of stock options granted by the Company and their weighted average exercise price

The following table summarizes our stock option activity and related information for the year ended December 31, 2018:

	Number of options		Weighted average exercise price		Weighted-Average Remaining Contractual Term (years)	Aggregate Intrinsic Value (millions)
Balance outstanding as of December 31, 2017	3,304,181		$	13.67
Granted	1,261,693		$	31.84
Exercised	(731,935	)	$	13.32
Canceled/forfeited	(150,153	)	$	22.83
Balance outstanding as of December 31, 2018	3,683,786		$	19.59	7.3	$	85.6
Options exercisable at December 31, 2018	1,910,058		$	16.05	6.1	$	50.4

Schedule of Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Table Text Block]

Total share-based compensation expense by statement of operations and comprehensive loss classification is presented below (in thousands):

	Year ended December 31,
	2018		2017		2016
Research and development expense	$	7,232	$	3,192	$	5,461
General and administrative expense	8,622		3,594		5,163
	$	15,854	$	6,786	$	10,624

Schedule of assumptions used to estimate the fair value of options granted at the date of grant using the Black-Scholes option pricing model

The assumptions used for the specified reporting periods and the resulting estimates of weighted-average estimated fair value per share of options granted during those periods are as follows:

	Year Ended December 31,
	2018	2017	2016
Risk-free interest rate	2.6%	2.1%	1.5%
Dividend yield	—%	—%	—%
Volatility factor	94.3%	96.0%	101.7%
Expected term (in years)	6.0	6.0	6.0
Weighted average estimated fair value per share	$24.39	$4.17	$13.32