Stock Plans (Tables)

6 Months Ended

Jun. 30, 2018

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Schedule of stock options valuation assumptions

The fair value of the stock options granted was estimated using the Black-Scholes valuation model with the following assumptions:

	Three and Six Months Ended June 30,
	2018		2017
Expected dividend yield	—	%	—	%
Expected volatility	54	%	59	%
Risk-free interest rate	2.95	%	1.87	%
Expected term (in years)	5.5		5.5

Schedule of restricted stock unit grants by type

For the three and six months endedJune 30, 2018 and 2017, RSU grants were composed of the following:

	Three Months Ended June 30,
	2018			2017
	Shares granted (in thousands)	Average grant date fair value		Shares granted (in thousands)	Average grant date fair value
Service-based	180	$	5.43	255	$	5.73
Performance objectives	—	n/a		158	5.73
Market-condition	20	6.56		31	7.89
Total RSUs granted	200	5.54		444	5.88
	Six Months Ended June 30,
	2018			2017
	Shares granted (in thousands)	Average grant date fair value		Shares granted (in thousands)	Average grant date fair value
Service-based	1,101	$	5.72	922	$	5.52
Performance objectives	78	5.85		158	5.73
Market-condition	355	7.49		334	7.27
Total RSUs granted	1,534	6.14		1,414	5.96

Schedule of allocation of stock-based compensation expense

Stock-based compensation cost included in cost of revenue; selling, general and administrative expense; and research and development expense is as follows (in thousands):

	Three Months Ended June 30,				Six Months Ended June 30,
	2018		2017		2018		2017
Cost of revenue	$	338	$	257	$	684	$	450
Selling, general and administrative	1,996		1,596		3,830		2,665
Research and development	413		401		857		677
Total stock-based compensation expense	$	2,747	$	2,254	$	5,371	$	3,792

Market condition restricted stock units

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Schedule of market-condition awards fair value assumptions

The fair value of the market-condition PSUs granted was calculated using a Monte Carlo valuation model with the following assumptions:

	Three and Six Months Ended June 30,
	2018		2017
Expected dividend yield	—	%	—	%
Expected volatility	41% - 47%		53	%
Risk-free interest rate	2.36% - 2.60%		1.55	%
Expected term (in years)	2.5 - 2.9		2.8

Restricted stock unit awards

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Schedule of RSU expense by vesting type

The following table summarizes the amount of compensation expense recognized for RSUs for the three and six months endedJune 30, 2018 and 2017 (in thousands):

	Three Months Ended June 30,				Six Months Ended June 30,
RSU Type	2018		2017		2018		2017
Service-based	$	1,102	$	669	$	2,186	$	1,337
Performance objectives	168		164		288		170
Market-condition	589		403		887		595
	$	1,859	$	1,236	$	3,361	$	2,102

Employee stock purchase plan

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Schedule of employee stock purchase plan fair value assumptions

The fair value of the ESPP shares was estimated using the Black-Scholes valuation model for a call and a put option with the following weighted-average assumptions:

	Three Months Ended June 30,						Six Months Ended June 30,
	2018			2017			2018			2017
Expected dividend yield	—		%	—		%	—		%	—		%
Expected volatility	40		%	29		%	42		%	29		%
Risk-free interest rate	1.74		%	0.62		%	1.57		%	0.62		%
Expected term (in years)	0.50			0.50			0.50			0.50
Fair value per share	$	1.34		$	1.19		$	1.32		$	1.19