Stock-based Compensation (Tables)

12 Months Ended

Dec. 31, 2018

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

Stock Option Activity and Related Information

A summary of the Company’s stock option activity under the Plans is as follows:

	Shares		Weighted- average exercise price per share		Weighted- average remaining contractual term (in years)	Aggregate intrinsic value (in thousands)
Outstanding at January 1,2018	5,196,253		$	13.32	6.98	$	3,861
Granted	1,186,944		8.69
Canceled and forfeited	(910,243	)	14.48
Exercised	(430,602	)	8.14
Outstanding at December 31, 2018	5,042,352		$	12.46	6.49	$	18,255
December 31, 2018:
Options vested and expected to vest	5,042,352		$	12.46	6.49	$	18,255
Options exercisable	3,381,664		$	12.48	5.49	$	12,099

Company's Options Outstanding

The following table summarizes information about the Company’s stock options outstanding at December 31, 2018:

	Outstanding		Exercisable
Exercise Price	Number of Shares	Weighted- Average Remaining Contractual Life in Years	Number of Shares	Weighted- Average Remaining Contractual Life in Years
$1.92	357,609	3.22	357,609	3.22
$2.24 – $6.72	344,531	2.62	336,231	2.46
$6.80 – $12.56	1,225,090	7.94	462,948	6.15
$12.77	484,976	5.83	463,659	5.82
$12.89 – $12.94	436,867	6.90	298,921	6.73
$13.01 – $14.95	322,713	7.22	223,186	6.61
$14.99 – $17.48	464,533	7.35	293,914	6.78
$17.83 – $18.90	1,150,432	6.49	777,347	5.62
$19.09 – $22.71	255,601	7.26	167,849	7.12
	5,042,352		3,381,664

Restricted Stock Units Award Activity

A summary of RSU activity under the Plans is as follows:

Non-vested RSUs	Share Equivalent		Weighted-Average Grant Date Fair Value
Non-vested at January 1, 2018	661,707		$	11.07
Changes during the year:
Granted	711,001		7.41
Vested	(109,469	)	14.82
Forfeited	(122,110	)	8.73
Non-vested at December 31, 2018	1,141,129		$	8.68

Stock Compensation Expense

The following table sets forth stock-based compensation expense related to stock-based arrangements under the Plans for the years ended December 31 as follows (in thousands):

	2018		2017		2016
Cost of revenue	$	616	$	719	$	548
Research and development	3,156		2,853		2,046
Selling, general and administrative	6,982		7,047		5,602
Total stock-based compensation expense	$	10,754	$	10,619	$	8,196

Fair Value of Employee Option Grant

The fair value of each employee option grant as of December 31 was estimated on the date of grant using the Black-Scholes option pricing model with the following assumptions:

	2018	2017	2016
Risk-free interest rates	2.22% - 3.01%	1.40% - 2.26%	1.18% - 2.12%
Expected term (years)	5.50 - 6.09	5.50 - 6.25	5.50 - 6.50
Expected dividend yield	—	—	—
Expected volatility	56.0% - 57.7%	53.9% - 58.0%	47.0%