Stockholders' Equity (Tables)

12 Months Ended

Dec. 31, 2020

Share-based Payment Arrangement [Abstract]

Schedule of share-based compensation expense

Share-based compensation expense is as follows (dollars in thousands):

	Years Ended December 31,
	2020		2019		2018
Research and development	$	2,431	$	2,599	$	1,943
Selling, general and administrative	14,130		12,247		9,348
Total	$	16,561	$	14,846	$	11,291

Schedule of assumptions used in estimation of fair value of each award option on the date of grant using Black-Scholes option-pricing model

The fair value of each option award is estimated on the date of the grant, using the Black-Scholes option-pricing model and the assumptions in the following table:

	Years Ended December 31,
	2020	2019	2018
Fair value of common stock	$21.13 - $23.99	$22.99 - $37.78	$37.20 - $58.15
Expected volatility	61.56% - 62.27%	61.36% - 63.28%	57.95% - 60.56%
Dividend yield	0%	0%	0%
Expected term	5.72 years - 6.54 years	5.53 years - 6.18 years	6.25 years
Risk-free interest rate	0.27% - 1.34%	1.69% - 2.55%	2.69% - 2.85%

Summary of stock option and SAR activity

The following table summarizes stock option and stock appreciation rights (SAR) activities:

	Number of Options	Weighted- Average Exercise Price		Weighted-Average Remaining Contractual Term (in years)	Aggregate Intrinsic Value (in thousands)
Outstanding , December 31, 2018	3,916,963	$	19.98	7.10	$	57,220
Granted	880,235	$	36.43
Exercised	(114,753)	$	12.90
Forfeited	(75,886)	$	34.80
Outstanding, December 31, 2019	4,606,559	$	23.06	6.66	$	27,716
Granted	1,370,225	$	23.55
Exercised	(204,373)	$	11.47
Forfeited	(320,549)	$	29.09
Outstanding, December 31, 2020	5,451,862	$	23.26	6.28	$	29,877
As of December 31, 2020
Vested and expected to vest	5,451,862	$	23.26	6.28	$	29,877
Exercisable	3,218,771	$	19.36	4.77	$	27,832