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Stock-Based Compensation (Tables)
 12 Months Ended
Dec. 31, 2019
Share-based Payment Arrangement [Abstract]  
Schedule of fair value assumptions
The fair value of each service-based stock option grant for the reported periods is estimated on the date of the grant using the Black-Scholes option-pricing model using the weighted average assumptions noted in the following table.
 
 
Year Ended December 31,
Service-Based Stock Options
 
2019
 
2018
 
2017
Expected dividend rate
 
—%
 
—%
 
—%
Expected stock price volatility
 
77.9-85.5%
 
82.3 – 88.3%
 
79.7 – 88.2%
Risk-free interest rate
 
1.4-2.7%
 
2.4 – 3.1%
 
1.39 – 2.3%
Expected life (years)
 
5.3 - 6.3
 
5.3 - 6.3
 
5.5 - 6.3
Summary of activity for service-based stock options
The following table summarizes the activity for service-based stock options for the indicated periods: 
Service-Based Stock Options
 
Options
 
Weighted Average
 Exercise Price
 
Weighted Average
 Remaining
 Contractual Term
(Years)
 
Aggregate
 Intrinsic
 Value
(Thousands)
Outstanding at December 31, 2018
 
4,790,683

 
$
5.85

 
7.7
 
$
11,407

Granted
 
2,033,760

 
$
17.51

 
 
 
 
Exercised
 
(1,197,016
)
 
$
3.64

 
 
 
 
Expired
 
(36,561
)
 
$
26.16

 
 
 
 
Forfeited
 
(537,916
)
 
$
11.26

 
 
 
 
Outstanding at December 31, 2019
 
5,052,950

 
$
10.35

 
7.7
 
$
37,974

Exercisable at December 31, 2019
 
2,385,824

 
$
7.49

 
6.5
 
$
25,113
Summary of activity for restricted stock awards
The following table summarizes the activity for restricted stock awards for the indicated periods: 
Restricted Stock Units
 
Number of Restricted Stock Awards
 
Weighted Average Grant Date Fair Value
 
Weighted Average Term
 
Aggregate Intrinsic Value
(Thousands)
Outstanding at December 31, 2018
 
—

 


 

 


Granted
 
186,922

 
$
17.71

 

 
$
3,310

Vested
 
—

 


 

 


Forfeited
 
(29,892
)
 
$
17.24

 

 


Unvested at December 31, 2019
 
157,030

 
$
17.80

 
1.6
 
$
2,732
Schedule of non-cash stock-based compensation expense
 
 
Years Ended December 31,
(in thousands)
 
2019
 
2018
 
2017
Cost of goods sold
 
$
2,029

 
$
1,015

 
$
428

Research and development
 
2,428

 
1,672

 
506

General, selling and administrative
 
8,722

 
4,536

 
1,746

Total non-cash stock-based compensation expense
 
$
13,179

 
$
7,223

 
$
2,680