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Stock-Based Compensation (Tables)
 12 Months Ended
Dec. 31, 2019
Share-based Payment Arrangement [Abstract]  
Schedule of Shares Available for Grant and Stock Option Activity Stock option activity under the Plans for the periods presented is as follows:
 
 
 
Options Outstanding
 
 
Shares
Available
for Grant
 
Shares
Subject to
Options
Outstanding
 
Weighted-
Average
Exercise
Price
 
Weighted-
Average
Remaining
Contractual
Life (Years)
 
Aggregate
Intrinsic
Value
 
 
(in thousands, except per share data and years)
 
Balance as of January 1, 2017
208


12,209


$
0.79


8.3
 
$
9,623

 
Shares authorized
8,661


—


—



 
 
 
Granted
(5,996
)

5,996


$
1.88



 
 
 
Exercised
—


(1,372
)

$
0.78



 
 
 
Forfeited
1,205


(1,205
)

$
0.87



 
 
 
Restricted stock awards granted
(1,064
)

—


—



 
 
 
Balance as of December 31, 2017
3,014

 
15,628

 
$
1.20

 
8.2
 
$
10,559

 
Shares authorized
3,196

 
—

 
—

 
 
 
 
 
Granted
(5,016
)
 
5,016

 
$
3.62

 
 
 
 
 
Exercised
—

 
(1,454
)
 
$
1.19

 
 
 
 
 
Forfeited
1,619

 
(1,619
)
 
$
2.25

 
 
 
 
 
Performance RSUs granted
(1,830
)
 
—

 
—

 
 
 
 
 
Restricted stock awards forfeited
754

 
—

 
—

 
 
 
 
 
Performance RSUs forfeited
4

 
—

 
—

 
 
 
 
 
Balance as of December 31, 2018
1,741

 
17,571

 
$
1.80

 
7.7
 
$
89,990

 
Shares authorized
10,504

 
—

 
—

 
 
 
 
 
Adjustment to plan
59


—


—

 
 
 
 
 
Exercised
—

 
(2,766
)
 
$
1.12

 
 
 
 
 
Forfeited or cancelled
785

 
(785
)
 
$
3.36

 
 
 
 
 
Restricted stock awards granted
(982
)
 
—

 
—

 
 
 
 
 
Restricted stock units, Performance RSUs and Performance stock units (PSUs) granted
(4,103
)
 
—

 
—

 
 
 
 
 
Restricted stock units, Performance RSUs and Performance stock units (PSUs) forfeited
121

 
—

 
—

 
 
 
 
 
Restricted stock units, Performance RSUs and Performance stock units (PSUs) returned to plan
35

 
—

 
—

 
 
 
 
 
Balance as of December 31, 2019
8,160

 
14,020

 
$
1.85

 
6.7
 
$
325,474

 
Vested and exercisable as of December 31, 2018
 
 
8,999

 
$
0.97

 
6.7
 
$
53,566

 
Vested and exercisable as of December 31, 2019
 
 
9,698

 
$
1.44

 
6.2
 
$
229,110

 
Schedule of Share-based Payment Award, Stock Options, Valuation Assumptions The following assumptions were used to calculate the fair value of these options and restricted stock units in the Monte Carlo simulation model at the grant dates:
 
Year Ended December 31,
 
2019
 
2018
Expected term (years)
10.0


9.6 - 10.0

Expected volatility
59.0
%

60.0% - 64.0%

Risk-free interest rate
2.8
%

2.6% - 2.9%

Dividend yield
—
%

—
%

The Black-Scholes option pricing model assumptions used in evaluating our awards to employees are as follows:
 
Year Ended December 31,
 
2019
 
2018
 
2017
Expected term (years)
n/a

6.0 - 6.8

6.3
Expected volatility
n/a

36.6% - 38.7%

37.1%
Risk-free interest rate
n/a

2.8% - 2.9%

2.0% - 2.3%
Dividend yield
n/a

—%

—%
Schedule of Restricted Stock Awards
Restricted Stock Awards
 
Shares
 
Weighted-
Average
Grant Date
Fair Value
 
(in thousands, except per share data)
Unvested balance, January 1, 2017
110


$
0.91

Issued
1,064


$
1.88

Vested
(47
)

$
0.83

Unvested balance, December 31, 2017
1,127

 
$
1.83

Issued
—

 
$
—

Vested
(373
)
 
$
1.73

Cancelled
(754
)
 
$
1.88

Unvested balance, December 31, 2018
—

 
$
—

Issued
982

 
$
9.76

Vested
(246
)
 
$
9.76

Unvested balance, December 31, 2019
736

 
$
9.76
Schedule of Restricted Stock Units
 
Restricted
Stock
Units, Performance
RSUs and PSUs
 
Weighted-
Average
Grant Date
Fair Value
 
(in thousands, except per
share data)
Unvested as of December 31, 2017
—

 
$
—

Granted
1,830

 
$
6.40

Vested
—

 
$
—

Forfeited
(3
)
 
$
3.92

Unvested as of December 31, 2018
1,827

 
$
6.42

Granted
4,102

 
$
12.49

Vested
(1,100
)
 
$
7.80

Forfeited
(121
)
 
$
9.28

Unvested as of December 31, 2019
4,708

 
$
11.31
Schedule of Share-based Payment Award, Employee Stock Purchase Plan, Valuation Assumptions
We estimated the fair value of ESPP purchase rights for our first offering period using a Black-Scholes option-pricing model with the following assumptions:
 
Year Ended December 31, 2019
Expected term (years)
0.77

Expected volatility
50.6
%
Risk-free interest rate
1.9
%
Dividend yield
—
%
Summary of Stock-Based Compensation Expense
Stock-based compensation expense in the consolidated statements of operations is summarized as follows:
 
Year Ended December 31,
 
2019
 
2018
 
2017
 
(in thousands)
Cost of revenue
$
151

 
$
18

 
$
—

Research and development expenses
8,182

 
2,188

 
541

Sales and marketing expenses
7,659

 
916

 
413

General and administrative expenses
16,640

 
3,210

 
1,164

Total stock-based compensation expense
$
32,632

 
$
6,332

 
$
2,118