Stock-Based Compensation (Tables)

12 Months Ended

Dec. 31, 2018

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Stock-Based Compensation Expense

The following table shows total stock-based compensation expense recognized in the accompanying consolidated statements of operations (in thousands):

	Year Ended December 31,
	2018		2017		2016
Research and development	$	8,249	$	5,031	$	6,463
General and administrative		6,428		4,058		8,594
Total stock-based compensation expense	$	14,677	$	9,089	$	15,057

Assumptions Used for Estimating Fair Value of Employee Stock Options

The assumptions used for estimating the fair value of the employee stock options are as follows:

	Year Ended December 31,
	2018			2017			2016
Risk-free interest rate	2.53-2.96%			1.81-2.28%			1.13-1.61%
Expected life of option (in years)	5.59-5.61			5.73-5.83			5.28-5.29
Expected dividend yield of stock		0	%		0	%		0	%
Expected volatility	0.72-0.75			0.71-0.72			0.68-0.70

Weighted-Average Assumptions Used for Estimating Fair Value of ESPP Purchase Rights

The weighted–average assumptions used for estimating the fair value of the ESPP purchase rights are as follows:

	Year Ended December 31,
	2018			2017			2016
Risk-free interest rate	2.16-2.80%			0.44-0.76%			0.41-0.80%
Expected life of option (in years)	0.5-2			0.5-2.0			0.5-2.0
Expected dividend yield of stock		0	%		0	%		0	%
Expected volatility	0.73-0.83			0.66-0.82			0.71-0.76