Stockholders' Equity (Tables)

9 Months Ended

Sep. 30, 2017

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Schedule of Stock-based Compensation Expense

Total stock-based compensation expense related to options, restricted stock awards, and ESPP for employees, non-employee directors, and non-employee consultants was allocated as follows (in thousands):

	Three Months Ended September 30,				Nine Months Ended September 30,
	2017		2016		2017		2016
Research and development	$	1,534	$	1,124	$	4,341	$	4,325
General and administrative	1,612		1,631		5,479		4,659
Total stock-based compensation expense	$	3,146	$	2,755	$	9,820	$	8,984

2014 Employee Stock Purchase Plan

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Fair Value Assumptions

The fair value of the option component of the shares purchased under the 2014 ESPP was estimated using the Black-Scholes option-pricing model with the following weighted-average assumptions:

	Three Months Ended   September 30,				Nine Months Ended September 30,
	2017		2016		2017		2016
Expected term (in years)	0.5		0.5		0.5		0.5
Expected volatility	46.6	%	80.9	%	59.1	%	72.0	%
Risk-free interest rate	1.1	%	0.4	%	0.9	%	0.4	%
Expected dividend rate	—	%	—	%	—	%	—	%

Employee Stock Option

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Fair Value Assumptions

The grant-date fair value of the employee stock options under the 2014 EIP and 2014 IN was estimated using the Black-Scholes option-pricing model with the following weighted-average assumptions:

	Three Months Ended September 30,				Nine Months Ended September 30,
	2017		2016		2017		2016
Expected term (in years)	6.0		6.0		6.0		6.0
Expected volatility	66.6	%	64.2	%	67.8	%	61.6	%
Risk-free interest rate	2.0	%	1.2	%	2.1	%	1.4	%
Expected dividend rate	—	%	—	%	—	%	—	%

Non-employee Stock Options

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Fair Value Assumptions

The fair value of the stock options outstanding for non-employee consultants is calculated at each reporting date using the Black-Scholes option pricing model with the following weighted-average assumptions:

	Three Months Ended September 30,				Nine Months Ended September 30,
	2017		2016		2017		2016
Expected term (in years)	8.9		7.2		8.9		7.5
Expected volatility	68.1	%	66.5	%	68.6	%	69.7	%
Risk-free interest rate	2.2	%	1.4	%	2.2	%	1.5	%
Expected dividend rate	—	%	—	%	—	%	—	%