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Stock-Based Compensation (Tables)
12 Months Ended
Dec. 31, 2020
Share-based Payment Arrangement [Abstract]  
Schedule of Stock-Based Compensation Expense
Stock-based compensation expense for restricted stock, restricted stock units (RSUs), performance-based restricted stock units (PSUs), stock options and purchase rights issued under our employee stock purchase plan was classified in the accompanying consolidated statements of operations as follows:
 Year Ended December 31,
 202020192018
 (in thousands)
Stock-based compensation expense:
Cost of revenue$4,298 $2,580 $1,692 
Research and development24,423 15,670 10,822 
Sales and marketing16,826 11,883 7,569 
General and administrative18,341 10,531 7,510 
Total stock-based compensation expense$63,888 $40,664 $27,593 
Summary of Restricted Stock and Restricted Stock Unit Activity Restricted stock, restricted stock unit and performance-based restricted stock unit activity during 2020, 2019 and 2018 was as follows:
 Restricted StockRSUs and PSUs
 Shares        Weighted-
Average Grant
Date Fair
Value
SharesWeighted-
Average
Grant Date
Fair Value
Unvested balance as of December 31, 2017210,083 $18.00 1,988,509 $14.77 
Granted— — 2,099,394 25.19 
Vested(187,706)18.80 (973,443)17.41 
Forfeited(700)23.01 (340,687)18.96 
Unvested balance as of December 31, 201821,677 10.88 2,773,773 21.21 
Granted— — 1,740,299 43.34 
Vested(21,677)10.88 (1,291,932)24.42 
Forfeited— — (285,216)26.14 
Unvested balance as of December 31, 2019— — 2,936,924 32.43 
Granted— — 1,725,531 57.57 
Vested— — (1,451,618)33.66 
Forfeited— — (268,923)40.56 
Unvested balance as of December 31, 2020— $— 2,941,914 $45.86 
Summary of Stock Option Activity
The following table summarizes information about stock option activity during the reporting periods:
Shares        Weighted
Average
Exercise
Price
Weighted
Average
Remaining
Contractual Life
(in years)
Aggregate
Intrinsic
Value
(in thousands)
Outstanding as of December 31, 20174,684,954 $9.68 
Granted107,850 24.44 
Exercised(944,658)8.05 $19,982 
Forfeited/cancelled(134,967)15.20 
Outstanding as of December 31, 20183,713,179 10.32 
Granted— — 
Exercised(968,057)10.55 $39,526 
Forfeited/cancelled(9,730)13.53 
Outstanding as of December 31, 20192,735,392 10.10 
Granted— — 
Exercised(783,645)9.98 $39,095 
Forfeited/cancelled(18,734)17.87 
Outstanding as of December 31, 20201,933,013 $10.07 4.05$154,816 
Vested and exercisable as of December 31, 20201,844,721 $9.75 3.94$148,339 
Summary of Share Based Compensation Valuation of Options Granted Assumptions
The following table reflects the assumptions used in the Black-Scholes option pricing model to calculate the expense related to the ESPP:
 Year Ended December 31,
 202020192018
Expected term (in years)
0.5 - 1.0
0.5 - 1.0
0.5 - 1.0
Expected volatility
47 - 53%
44 - 55%
37%
Risk-free interest rate
0.1 – 0.3%
1.9 – 2.5%
2.0 – 2.6%
Expected dividend yield— — — 
Grant date fair value per share
$9.63 – 22.30
$14.17 –17.94
$6.62 – 10.95