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Stock Plans (Tables)
 9 Months Ended
Sep. 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 Months Ended September 30,
 
Nine Months Ended September 30,
 
 
2017
 
2018
 
2017
Expected dividend yield
 
—
%
 
—
%
 
—
%
Expected volatility
 
58
%
 
54
%
 
58% - 59%

Risk-free interest rate
 
1.96
%
 
2.95
%
 
1.87% - 1.96%

Expected term (in years)
 
5.5

 
5.5

 
5.5
Schedule of restricted stock unit grants by type
For the three and nine months ended September 30, 2018 and 2017, RSU grants were composed of the following:
 
 
Three Months Ended September 30,
 
 
2018
 
2017
 
 
Shares granted
(in thousands)
 
Average grant date fair value
 
Shares granted
(in thousands)
 
Average grant date fair value
Service-based
 
10

 
$
3.69

 
321

 
$
5.58

Performance objectives
 
—

 
n/a

 
—

 
n/a

Market-condition
 
—

 
n/a

 
34

 
6.79

Total RSUs granted
 
10

 
3.69

 
355

 
5.70

 
 
 
 
 
 
 
 
 
 
 
Nine Months Ended September 30,
 
 
2018
 
2017
 
 
Shares granted
(in thousands)
 
Average grant date fair value
 
Shares granted
(in thousands)
 
Average grant date fair value
Service-based
 
1,111

 
$
5.70

 
1,242

 
$
5.54

Performance objectives
 
78

 
5.85

 
158

 
5.73

Market-condition
 
355

 
7.49

 
368

 
7.22

Total RSUs granted
 
1,544

 
6.12

 
1,768

 
5.91
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 September 30,
 
Nine Months Ended September 30,
 
 
2018
 
2017
 
2018
 
2017
Cost of revenue
 
$
255

 
$
271

 
$
939

 
$
721

Selling, general and administrative
 
1,479

 
2,097

 
5,309

 
4,762

Research and development
 
260

 
387

 
1,117

 
1,064

Total stock-based compensation expense
 
$
1,994

 
$
2,755

 
$
7,365

 
$
6,547
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 Nine Months Ended September 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 nine months ended September 30, 2018 and 2017 (in thousands):
 
 
Three Months Ended September 30,
 
Nine Months Ended September 30,
RSU Type
 
2018
 
2017
 
2018
 
2017
Service-based
 
$
1,043

 
$
1,124

 
$
3,229

 
$
2,460

Performance objectives
 
(49
)
 
219

 
239

 
389

Market-condition
 
526

 
432

 
1,413

 
1,027

 
 
$
1,520

 
$
1,775

 
$
4,881

 
$
3,876
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 September 30,
 
Nine Months Ended September 30,
 
 
2018
 
2017
 
2018
 
2017
Expected dividend yield
 
—
%
 
—
%
 
—
%
 
—
%
Expected volatility
 
38
%
 
37
%
 
40
%
 
32
%
Risk-free interest rate
 
2.09
%
 
1.09
%
 
1.74
%
 
0.78
%
Expected term (in years)
 
0.50

 
0.38

 
0.50

 
0.46

Fair value per share
 
$
1.39

 
$
1.45

 
$
1.34

 
$
1.28