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Stock Plans (Tables)
 9 Months Ended
Sep. 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 nine months ended September 30, 2017 was estimated using the Black-Scholes valuation model with the following assumptions:
 
 
Three Months Ended September 30, 2017
 
Nine Months Ended September 30, 2017
Expected dividend yield
 
—
%
 
—
%
Expected volatility
 
58
%
 
58% - 59%

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

Expected term (in years)
 
5.5

 
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 September 30,
 
Nine Months Ended September 30,
 
 
2017
 
2016
 
2017
 
2016
Cost of revenue
 
$
271

 
$
146

 
$
721

 
$
643

Selling, general and administrative
 
2,097

 
799

 
4,762

 
2,436

Research and development
 
387

 
152

 
1,064

 
680

Total stock-based compensation expense
 
$
2,755

 
$
1,097

 
$
6,547

 
$
3,759
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,
 
 
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
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, 2017 and 2016 (in thousands):
 
 
Three Months Ended September 30,
 
Nine Months Ended September 30,
RSU Type
 
2017
 
2016
 
2017
 
2016
Service-based
 
$
1,124

 
$
544

 
$
2,460

 
$
1,613

Performance objectives
 
219

 
30

 
389

 
71

Market-condition
 
432

 
250

 
1,027

 
621

 
 
$
1,775

 
$
824

 
$
3,876

 
$
2,305
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 nine months ended September 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 Months Ended September 30,
 
Nine Months Ended September 30,
 
 
2017
 
2016
 
2017
 
2016
Expected dividend yield
 
—
%
 
—
%
 
—
%
 
—
%
Expected volatility
 
37
%
 
54
%
 
32
%
 
58
%
Risk-free interest rate
 
1.09
%
 
0.36
%
 
0.78
%
 
0.45
%
Expected term (in years)
 
0.38

 
0.50

 
0.46

 
0.50

Fair value per share
 
$
1.45

 
$
1.59

 
$
1.28

 
$
2.04