Stock-Based Compensation (Tables)

12 Months Ended

Dec. 31, 2018

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

Compensation Expense Related to Stock-Based Compensation Plans

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

	Year ended December 31,
	2018		2017		2016
Cost of sales	$	763	$	579	$	617
Research and development	2,266		1,886		1,551
Sales and marketing	2,843		2,129		1,189
General and administrative	5,837		4,467		4,629
Total stock-based compensation expense	$	11,709	$	9,061	$	7,986

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:

	Year ended December 31,
	2018		2017		2016
Risk-free interest rate	2.49	%	2.30	%	1.47	%
Expected option life (in years)	6.29		6.63		6.59
Volatility rate	36	%	36	%	36	%
Dividend rate	—	%	—	%	—	%

Summary of Status of Stock Option Activity

A summary of the status of stock option activity for the years ended December 31, 2016, 2017 and 2018 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, 2016	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
Granted	159		46.50
Exercised	(891	)	17.07
Cancelled	(50	)	21.19
Outstanding at December 31, 2018	1,877		$	21.53	5.98	$	49,976
Vested and expected to vest at December 31, 2018	1,838		$	21.34	5.93	$	49,302
Exercisable at December 31, 2018	1,117		$	19.28	4.83	$	32,374

Summary of Status of Stock Awards Activity

A summary of the status of restricted stock unit activity for the years ended December 31, 2016, 2017 and 2018 is as follows (in thousands, except price data):

	Shares		Weighted-average grant date fair value
Non-vested at January 1, 2016	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
Granted	242		49.97
Vested	(296	)	21.70
Forfeited	(16	)	28.40
Non-vested at December 31, 2018	676		$	30.75