Stock-Based Compensation (Tables)

12 Months Ended

Dec. 31, 2018

Disclosure of Compensation Related Costs, Share-based Payments [Abstract]

Schedule of non-cash stock-based compensation expense

Non-cash stock-based compensation expense (employee stock purchase plan and service-based stock options) is summarized in the following table: 

	Years Ended December 31,
(in thousands)	2018		2017		2016
Cost of goods sold	$	1,015	$	428	$	427
Research and development	1,672		506		497
General, selling and administrative	4,536		1,746		1,575
Total non-cash stock-based compensation expense	$	7,223	$	2,680	$	2,499

Schedule of weighted average assumptions used to estimate fair value of each service-based stock option grant

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	2018	2017	2016
Expected dividend rate	—%	—%	—%
Expected stock price volatility	82.3-88.3%	79.7 – 88.2%	78.7 – 92.2%
Risk-free interest rate	2.4-3.1%	1.39 – 2.3%	1.1 – 2.1%
Expected life (years)	5.3 - 6.3	5.5 - 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	Aggregate Intrinsic Value
Outstanding at December 31, 2017	4,528,426		$	3.77	8.0	$	10,776,000
Granted	1,644,160		$	9.64
Exercised	(1,180,815	)	$	3.14
Expired	(12,862	)	$	6.84
Forfeited	(188,226	)	$	5.89
Outstanding at December 31, 2018	4,790,683		$	5.85	7.7	$	11,407
Exercisable at December 31, 2018	2,292,065		$	5.40		$	4,840