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Stock Plans (Tables)
 6 Months Ended
Jun. 30, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Schedule of stock options fair value assumptions
The fair value of the stock options granted during the three and six months ended June 30, 2017 was estimated using the Black-Scholes valuation model with the following assumptions:
 
 
Three and Six Months Ended June 30, 2017
Expected dividend yield
 
—
%
Expected volatility
 
59
%
Risk-free interest rate
 
1.87
%
Expected term (in years)
 
5.5
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,
 
 
2017
 
2016
 
2017
 
2016
Cost of revenue
 
$
257

 
$
262

 
$
450

 
$
497

Selling, general and administrative
 
1,596

 
966

 
2,665

 
1,637

Research and development
 
401

 
231

 
677

 
528

Total stock-based compensation expense
 
$
2,254

 
$
1,459

 
$
3,792

 
$
2,662
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 ended June 30, 2017 and 2016 (in thousands):
 
 
Three Months Ended June 30,
 
Six Months Ended June 30,
RSU Type
 
2017
 
2016
 
2017
 
2016
Service-based
 
$
669

 
$
536

 
$
1,337

 
$
1,069

Performance objectives
 
164

 
29

 
170

 
41

Market-condition
 
403

 
180

 
595

 
371

 
 
$
1,236

 
$
745

 
$
2,102

 
$
1,481
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,
 
 
2017
 
2016
Expected dividend yield
 
—
%
 
—
%
Expected volatility
 
53
%
 
62
%
Risk-free interest rate
 
1.55
%
 
1.07
%
Expected term (in years)
 
2.8

 
3.0
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 for the three and six months ended June 30, 2017 and 2016 was estimated using the Black-Scholes valuation model for a call and a put option with the following weighted-average assumptions:
 
 
Three and Six Months Ended June 30,
 
 
2017
 
2016
Expected dividend yield
 
—
%
 
—
%
Expected volatility
 
29
%
 
60
%
Risk-free interest rate
 
0.62
%
 
0.49
%
Expected term (in years)
 
0.5

 
0.5

Fair value per share
 
$
1.19

 
$
2.27