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Stock-Based Compensation (Tables) | 12 Months Ended
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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,
| 2020 |  | 2019 |  | 2018
| (in thousands)
Stock-based compensation expense: |  |  |  |  |
Cost of revenue | $ | 4,298 |  |  | $ | 2,580 |  |  | $ | 1,692 |
Research and development | 24,423 |  |  | 15,670 |  |  | 10,822 |
Sales and marketing | 16,826 |  |  | 11,883 |  |  | 7,569 |
General and administrative | 18,341 |  |  | 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 Stock |  | RSUs and PSUs
| Shares |  | Weighted- Average Grant Date Fair Value |  | Shares |  | Weighted- Average Grant Date Fair Value
Unvested balance as of December 31, 2017 | 210,083 |  |  | $ | 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, 2018 | 21,677 |  |  | 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, 2017 | 4,684,954 |  |  | $ | 9.68 |  |  |  |  |
Granted | 107,850 |  |  | 24.44 |  |  |  |  |
Exercised | (944,658) |  |  | 8.05 |  |  |  |  | $ | 19,982 |
Forfeited/cancelled | (134,967) |  |  | 15.20 |  |  |  |  |
Outstanding as of December 31, 2018 | 3,713,179 |  |  | 10.32 |  |  |  |  |
Granted | — |  |  | — |  |  |  |  |
Exercised | (968,057) |  |  | 10.55 |  |  |  |  | $ | 39,526 |
Forfeited/cancelled | (9,730) |  |  | 13.53 |  |  |  |  |
Outstanding as of December 31, 2019 | 2,735,392 |  |  | 10.10 |  |  |  |  |
Granted | — |  |  | — |  |  |  |  |
Exercised | (783,645) |  |  | 9.98 |  |  |  |  | $ | 39,095 |
Forfeited/cancelled | (18,734) |  |  | 17.87 |  |  |  |  |
Outstanding as of December 31, 2020 | 1,933,013 |  |  | $ | 10.07 |  |  | 4.05 |  | $ | 154,816 |
Vested and exercisable as of December 31, 2020 | 1,844,721 |  |  | $ | 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,
| 2020 |  | 2019 |  | 2018
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

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- Definition Tabular disclosure of the significant assumptions used during the year to estimate the fair value of stock options, including, but not limited to: (a) expected term of share options and similar instruments, (b) expected volatility of the entity's shares, (c) expected dividends, (d) risk-free rate(s), and (e) discount for post-vesting restrictions. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (f)(2) -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 + Details
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- Definition Disclosure of the number and weighted-average grant date fair value for restricted stock and restricted stock units that were outstanding at the beginning and end of the year, and the number of restricted stock and restricted stock units that were granted, vested, or forfeited during the year. + References No definition available. + Details
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