Stockholders' Equity (Tables)

12 Months Ended

Dec. 31, 2019

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,
	2019		2018		2017
Research and development	$	2,599	$	1,943	$	1,387
Selling, general and administrative	12,247		9,348		7,046
Total	$	14,846	$	11,291	$	8,433

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,
	2019	2018	2017
Fair value of common stock	$22.99 - $37.78	$37.20 - $58.15	$25.30 - $41.00
Expected volatility	61.36% - 63.28%	57.95% - 60.56%	53.61% - 60.60%
Dividend yield	0%	0%	0%
Expected term	5.53 years - 6.18 years	6.25 years	6.25 years
Risk-free interest rate	1.69% - 2.55%	2.69% - 2.85%	1.90% - 2.18%
Expected forfeiture rate	0%	0%	0%

Summary of stock option and SAR activity

	Number of Options		Weighted- Average Exercise Price		Weighted-Average Remaining Contractual Term (in years)	Aggregate Intrinsic Value (in thousands)
Outstanding , December 31, 2017	4,280,670		$	14.50	7.37	$	108,520
Granted	762,915		$	39.91
Exercised	(930,483	)	$	10.07		$	36,317
Forfeited	(196,139	)	$	25.01
Outstanding, December 31, 2018	3,916,963		$	19.98	7.10	$	57,220
Granted	880,235		$	36.43
Exercised	(114,753	)	$	12.90		$	2,423
Forfeited	(75,886	)	$	34.80
Outstanding, December 31, 2019	4,606,559		$	23.05	6.66	$	27,716
As of December 31, 2019
Vested and expected to vest	4,606,559		$	23.05	6.66	$	27,716
Exercisable	2,598,112		$	15.68	5.48	$	25,594