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Stockholders' Equity (Tables)
6 Months Ended
Jun. 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
June 30,
 
Six Months Ended
June 30,
2017
 
2016
 
2017
 
2016
Research and development
$
1,504

 
$
1,797

 
$
2,808

 
$
3,201

General and administrative
2,015

 
1,455

 
3,866

 
3,028

Total stock-based compensation expense
$
3,519

 
$
3,252

 
$
6,674

 
$
6,229

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 and Six Months Ended 
 June 30,
 
2017
 
2016
Expected term (in years)
0.5

 
0.5

Expected volatility
72.4
%
 
50.9
%
Risk-free interest rate
0.7
%
 
0.5
%
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
June 30,
 
Six Months Ended
June 30,
 
2017
 
2016
 
2017
 
2016
Expected term (in years)
6.0

 
5.7

 
6.0

 
6.0

Expected volatility
67.0
%
 
61.9
%
 
68.1
%
 
61.2
%
Risk-free interest rate
2.0
%
 
1.4
%
 
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 granted to 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
June 30,
 
Six Months Ended
June 30,
 
2017
 
2016
 
2017
 
2016
Expected term (in years)
9.0

 
7.5

 
9.0

 
7.6

Expected volatility
70.0
%
 
69.2
%
 
70.0
%
 
71.3
%
Risk-free interest rate
2.2
%
 
1.5
%
 
2.2
%
 
1.6
%
Expected dividend rate
%
 
%
 
%
 
%