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Stock-Based Compensation (Tables)
12 Months Ended
Dec. 31, 2018
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]  
Summary of Fair Value Assumptions on Stock Options Granted
For the years ended December 31, 2018, 2017 and 2016, the fair value of stock options granted to employees was estimated on the grant date using the Black-Scholes valuation model with the following assumptions:
 
2018
 
2017
 
2016
Dividend yield
0
%
 
0
%
 
0
%
Expected term (in years)
5.2 - 6.1

 
5.3 - 6.3

 
6.08

Risk-free interest rates
2.5% - 2.9%

 
1.9% - 2.2%

 
1.2% - 2.1%

Expected volatility
38% - 45%

 
39% - 43%

 
42% - 45%

Summary of Activity under Stock Option Plans
The following table summarizes activity under the Company’s stock option plans:
 
 
Number of Shares Underlying Outstanding Options
 
Weighted-Average
Exercise Price
 
Weighted-Average Remaining Contractual Term (Years)
 
Aggregate Intrinsic Value (in thousands)
Balances at January 1, 2016
 
20,710,510

 
$
2.79

 
8.28
 
$
8,634

Granted
 
2,624,450

 
3.20

 
 
 
 
Exercised
 
(272,591
)
 
0.99

 
 
 
 
Forfeited and canceled
 
(2,132,867
)
 
2.93

 
 
 
 
Balances at December 31, 2016
 
20,929,502

 
2.85

 
7.58
 
11,149

Granted
 
5,803,596

 
3.58

 
 
 
 
Exercised
 
(1,554,944
)
 
1.64

 
 
 
 
Forfeited and canceled
 
(1,570,408
)
 
2.98

 
 
 
 
Balances at December 31, 2017
 
23,607,746

 
3.10

 
7.39
 
22,260

Granted
 
4,468,523

 
5.88

 
 
 
 
Exercised
 
(3,522,631
)
 
2.32

 
 
 
 
Forfeited and canceled
 
(779,359
)
 
3.80

 
 
 
 
Balances at December 31, 2018
 
23,774,279

 
3.71

 
7.10
 
342,262

Vested and exercisable as of December 31, 2018
 
13,774,468

 
3.19

 
6.13
 
205,455

Vested and expected to vest as of December 31, 2018
 
23,774,279

 
3.71

 
7.10
 
342,262

Summary of Activity under RSU Plans
The following table summarizes the RSU activity and related information under the 2018 EIP:
 
Number of
RSUs Outstanding
 
Weighted-Average
Grant Date Fair Value
Unvested balance - January 1, 2018

 
$

Granted
327,202

 
15.00

Vested
(38,742
)
 
15.00

Forfeited/canceled

 

Unvested balance - December 31, 2018
288,460

 
$
15.00

Summary of Fair Value Assumptions on Employee Stock Purchas Plans
For the year ended December 31, 2018, the assumptions used to determine the fair value of the shares to be awarded are as follows:
 
2018
Dividend yield
0
%
Expected term (in years)
0.5 - 2.0

Risk-free interest rates
2.4% - 2.9%

Unvested balance - expected volatility
37
%
Summary of Components of Stock-Based Compensation Expense Recognized
The following table summarizes the components of stock-based compensation expense recognized in the consolidated statements of operations for the years ended December 31, 2018, 2017 and 2016 (in thousands):
 
2018
 
2017
 
2016
Cost of revenue
$
282

 
$
290

 
$
193

Research and development
3,258

 
1,797

 
1,820

Sales and marketing
1,637

 
1,299

 
1,052

General and administrative
5,184

 
3,460

 
4,201

Total
$
10,361

 
$
6,846

 
$
7,266