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Share-Based Compensation (Tables)
 12 Months Ended
Dec. 31, 2018
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]  
Stock Option Activity
A summary of Common Stock option activity is as follows:
 
Number of Options
(in thousands)
 
Weighted Average Exercise Price
 
Weighted Average Remaining Contractual Term
(years)
 
Aggregate Intrinsic Value
(in thousands)
Outstanding at December 31, 2016
2,321

 
$
1.44

 
 
 
 
Granted
989

 
$
11.37

 
 
 
 
Exercised
(412
)
 
$
0.74

 
 
 
 
Forfeited
(35
)
 
$
11.36

 
 
 
 
Outstanding at December 31, 2017
2,863

 
$
4.85

 
 
 
 
Granted
1,108

 
$
7.37

 
 
 
 
Exercised
(280
)
 
$
0.65

 
 
 
 
Forfeited or canceled
(164
)
 
$
9.52

 
 
 
 
Outstanding at December 31, 2018
3,527

 
$
5.76

 
6.95
 
$
2,698

Vested or expected to vest at December 31, 2018
3,527

 
$
5.76

 
6.95
 
$
2,698

Exercisable as of December 31, 2018
1,979

 
$
4.20

 
5.61
 
$
2,421
Fair Value Assumptions for Stock Options
The fair value was determined by the Black-Scholes option pricing model using the following weighted-average assumptions:
 
Year Ended
December 31,
 
2018
 
2017
Expected term, in years
6.35

 
6.43

Expected volatility
85.2
%
 
83.8
%
Risk-free interest rate
2.6
%
 
2.1
%
Expected dividend yield
—
%
 
—
%
Weighted-average exercise price
$
7.37

 
$
11.37
Allocation of Share-based Compensation Expense
Share-based compensation related to all equity awards issued pursuant to the 2008 Plan and 2016 Plan and for estimated shares to be issued under the ESPP for the purchase periods active during each respective period is included in the consolidated statements of operations and comprehensive loss as follows:
 
Year Ended
December 31,
 
2018
 
2017
 
(in thousands)
Research and development
$
1,286

 
$
917

General and administrative
2,393

 
1,492

Total share-based compensation expense
$
3,679

 
$
2,409