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Share-Based Compensation (Tables) | 12 Months Ended
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Dec. 31, 2020
Share-based Payment Arrangement [Abstract] |
Summary of shares available for grant | The following table summarizes the shares available for grant under the 2018 Plan:
| Shares Available for Grant
Balance - December 31, 2019 | 3,297,638 |
Authorized | 2,144,521 |
Granted | (512,945) |
Shares withheld to cover taxes | 16,742 |
Forfeited | 75,314 |
Balance - December 31, 2020 | 5,021,270 |
Schedule of fair value assumptions | Following are the assumptions used in the Black-Scholes valuation model for options granted during the periods shown below:
| Year Ended December 31,
| 2020 |  | 2019 |  | 2018
Risk-free interest rate | 1.3% |  | 2.3% |  | 2.8%
Average expected term (years) | 7.0 |  | 6.1 |  | 5.8
Expected volatility | 55.0% |  | 55.0% |  | 55.0%
Dividend yield | — |  | — |  | —
Schedule of stock option activity | The following table summarizes the Company’s stock option activity during the period from December 31, 2017 through December 31, 2020:
| Number of Stock Options |  | Weighted Average Exercise Price |  | Weighted Average Remaining Contractual Life (Years) |  | Aggregate Intrinsic Value (in thousands)(1)
Outstanding at December 31, 2017 | 4,207,029 |  |  | $ | 0.88 |  |  | 7.2 |  | $ | 8,936 |
Granted | 2,136,012 |  |  | $ | 6.49 |  |  | — |  | $ | — |
Exercised | (1,139,962) |  |  | $ | 1.20 |  |  | — |  | $ | 5,722 |
Cancelled/Forfeited | (82,786) |  |  | $ | 2.03 |  |  | — |  | $ | — |
Outstanding at December 31, 2018 | 5,120,293 |  |  | $ | 3.13 |  |  | 7.3 |  | $ | 81,371 |
Granted | 1,571,925 |  |  | $ | 39.01 |  |  | — |  | $ | — |
Exercised | (1,429,756) |  |  | $ | 1.87 |  |  | — |  | $ | 121,591 |
Cancelled/Forfeited | (91,486) |  |  | $ | 9.33 |  |  | — |  | $ | — |
Outstanding at December 31, 2019 | 5,170,976 |  |  | $ | 14.28 |  |  | 7.5 |  | $ | 329,879 |
Granted | 268,193 |  |  | $ | 99.74 |  |  | — |  | $ | — |
Exercised | (1,163,374) |  |  | $ | 7.75 |  |  | — |  | $ | 132,935 |
Cancelled/Forfeited | (57,517) |  |  | $ | 37.14 |  |  | — |  | $ | — |
Outstanding at December 31, 2020 | 4,218,278 |  |  | $ | 21.20 |  |  | 6.6 |  | $ | 443,595 |
Vested and exercisable at December 31, 2020 | 2,634,631 |  |  | $ | 9.74 |  |  | 5.7 |  | $ | 305,890 |
Vested and expected to vest at December 31, 2020 | 3,869,597 |  |  | $ | 18.26 |  |  | 6.5 |  | $ | 417,857 |
__________ (1) Aggregate intrinsic value is calculated as the difference between the value of common stock on the transaction date and the exercise price multiplied by the number of shares issuable under the stock option. Aggregate intrinsic value of shares outstanding at the beginning and end of the reporting period is calculated as the difference between the value of common stock on the beginning and end dates, respectively, and the exercise price multiplied by the number of shares outstanding.
Schedule of RSU activity | The following table summarizes the Company’s RSU activity from January 1, 2019 through December 31, 2020:
|  | Number of Units |  | Weighted Average Grant Date Fair Value Per Unit
Unvested at January 1, 2019 |  | — |  |  | $ | — |
Granted |  | 173,196 |  |  | $ | 126.29 |
Vested |  | (23,552) |  |  | $ | 84.84 |
Cancelled/Forfeited |  | (640) |  |  | $ | — |
Unvested at December 31, 2019 |  | 149,004 |  |  | $ | 132.73 |
Granted |  | 244,752 |  |  | $ | 109.61 |
Vested |  | (99,970) |  |  | $ | 127.71 |
Cancelled/Forfeited |  | (17,797) |  |  | $ | — |
Unvested at December 31, 2020 |  | 275,989 |  |  | $ | 114.99 |
Schedule of restricted stock activity | The following table summarizes the Company’s restricted stock activity:
| Number of Shares of Restricted Stock |  | Weighted Average Remaining Contractual Life (Years) |  | Weighted Average Grant Date Fair Value Per Share
Unvested at December 31, 2017 | — |  |  | — |  | $ | — |
Granted | 135,791 |  |  | — |  | $ | 17.03 |
Vested/Released | (35,664) |  |  | — |  | $ | 17.03 |
Cancelled/Forfeited | — |  |  | — |  | $ | — |
Unvested at December 31, 2018 | 100,127 |  |  | 1.6 |  | $ | 17.03 |
Granted | 99,433 |  |  | — |  | $ | 20.02 |
Vested/Released | (87,239) |  |  | — |  | $ | 19.21 |
Cancelled/Forfeited | (23,333) |  |  | — |  | $ | — |
Unvested at December 31, 2019 | 88,988 |  |  | 1.2 |  | $ | 19.49 |
Granted | — |  |  | — |  | $ | — |
Vested/Released | (76,804) |  |  | — |  | $ | 19.97 |
Cancelled/Forfeited | — |  |  | — |  | $ | — |
Unvested at December 31, 2020 | 12,184 |  |  | 0.3 |  | $ | 20.02 |

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