Stock-based Compensation (Tables)

12 Months Ended

Dec. 31, 2017

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 1,2017	4,721,941		$	12.00	7.41	$	48,670
Granted	1,180,086		17.90
Canceled and forfeited	(478,078	)	15.90
Exercised	(227,696	)	4.78
Outstanding at December 31, 2017	5,196,253		$	13.32	6.98	$	3,861
December 31, 2017:
Options vested and expected to vest	4,792,694		$	13.07	6.84	$	3,861
Options exercisable	3,437,421		$	11.90	6.19	$	3,861

Company's Options Outstanding

The following table summarizes information about the Company’s stock options outstanding at December 31, 2017:

	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	431,345	4.21	431,345	4.21
$2.24 – $6.72	494,155	3.87	494,155	3.87
$7.47 – $12.56	480,694	6.76	384,717	6.28
$12.77	674,356	7.06	488,124	7.04
$12.94	577,212	8.05	276,718	7.99
$13.01 – $14.95	360,230	7.62	241,553	7.42
$14.99 – $17.50	531,282	8.27	218,634	7.36
$17.89 – $18.90	1,346,556	7.63	806,525	6.66
$19.09 – $22.71	300,423	8.22	95,650	7.33
	5,196,253		3,437,421

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, 2017	163,602		$	15.70
Changes during the year:
Granted	603,493		10.73
Vested	(66,956	)	15.77
Forfeited	(38,432	)	17.20
Non-vested at December 31, 2017	661,707		$	11.07

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):

	2017		2016		2015
Cost of revenue	$	719	$	548	$	471
Research and development	2,853		2,046		1,453
Selling, general and administrative	7,047		5,602		3,919
Total stock-based compensation expense	$	10,619	$	8,196	$	5,843

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:

	2017	2016	2015
Risk-free interest rates	1.40% - 2.26%	1.18% - 2.12%	1.37% - 1.97%
Expected term (years)	5.50 - 6.25	5.50 - 6.50	6.25
Expected dividend yield	—	—	—
Expected volatility	53.9% - 58.0%	47.0%	57.0%