Stock-Based Compensation (Tables)

12 Months Ended

Dec. 31, 2017

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

Compensation Expense Related to Stock-Based Compensation Plans

tock-based compensation expense related to stock options and restricted stock units was as follows (in thousands):

	Year ended December 31,
	2017		2016		2015
Cost of sales	$	579	$	617	$	581
Research and development	1,886		1,551		734
Sales and marketing	2,129		1,189		1,554
General and administrative	4,467		4,629		4,550
Total stock-based compensation expense	$	9,061	$	7,986	$	7,419

Estimated Fair Value of Each Stock Option Award

The estimated fair value of each stock option was determined on the date of grant using the Black-Scholes option valuation model with the following weighted-average assumptions for the option grants:

	Year ended December 31,
	2017		2016		2015
Risk-free interest rate	2.30	%	1.47	%	1.50	%
Expected option life (in years)	6.63		6.59		6.24
Volatility rate	36	%	36	%	40	%
Dividend rate	—	%	—	%	—	%

Summary of Status of Stock Option Activity

A summary of the status of stock option activity for the years ended December 31, 2015, 2016 and 2017 is as follows (in thousands, except price data and years):

	Number of Shares		Weighted- average exercise price per share		Weighted- average remaining contractual term (in years)	Aggregate intrinsic value
Outstanding at January 1, 2015	3,607		$	16.37
Granted	659		23.15
Exercised	(168	)	12.30
Cancelled	(131	)	23.41
Outstanding at December 31, 2015	3,967		17.44
Granted	677		15.48
Exercised	(553	)	13.76
Cancelled	(150	)	20.86
Outstanding at December 31, 2016	3,941		17.49
Granted	263		22.21
Exercised	(1,527	)	16.38
Cancelled	(18	)	24.91
Outstanding at December 31, 2017	2,659		$	18.54	5.69	$	65,969
Vested and expected to vest at December 31, 2017	2,574		$	18.51	5.61	$	63,914
Exercisable at December 31, 2017	1,463		$	17.71	3.99	$	37,503
Available for future grant at December 31, 2017	1,848

Summary of Status of Stock Awards Activity

A summary of the status of stock awards activity for the years ended December 31, 2015, 2016 and 2017 is as follows (in thousands, except price data):

	Shares		Weighted-average grant date fair value
Non-vested at January 1, 2015	402		$	14.84
Granted	171		22.79
Vested	(96	)	18.01
Forfeited	(18	)	22.87
Non-vested at December 31, 2015	459		21.61
Granted	185		16.14
Vested	(120	)	18.50
Forfeited	(23	)	20.80
Non-vested at December 31, 2016	501		20.37
Granted	349		22.34
Vested	(100	)	23.49
Forfeited	(4	)	18.69
Non-vested at December 31, 2017	746		$	20.88