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Stock-Based Compensation (Tables) | 3 Months Ended
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Mar. 31, 2020
Disclosure of Compensation Related Costs, Share-based Payments [Abstract] |
Summary of Stock Option Activity Under Plan | The following summarizes option activity under the 2018 Plan:
| Outstanding Options |  |  |  |  |  |
| Number of Options |  | Weighted- Average Exercise Price |  | Weighted- Average Remaining Contract Term |  | Aggregate intrinsic value
|  |  |  |  | (in years) |  | (in thousands)
Balance, December 31, 2019 | 9,190,522 |  |  | $ | 14.51 |  |  | 8.82 |  | $ | 110,490 |
Granted | 2,675,672 |  |  | 18.38 |  |  |  |  |  |
Exercised | (105,895) |  |  | 2.87 |  |  |  |  |  |
Forfeited | (132,910) |  |  | 9.58 |  |  |  |  |  |
Balance, March 31, 2020 | 11,627,389 |  |  | $ | 15.56 |  |  | 8.87 |  | $ | 77,096 |
Exercisable, March 31, 2020 | 6,716,452 |  |  | $ | 13.66 |  |  | 8.90 |  | $ | 51,873 |
Vested and expected to vest, March 31, 2020 | 11,627,389 |  |  | $ | 15.56 |  |  | 8.87 |  | $ | 77,096 |
Schedule Stock Option Valuation Assumptions | The fair value of employee, consultant and director stock option awards was estimated at the date of grant using a Black-Scholes option-pricing model with the following assumptions:
| Three Months Ended March 31, |  |
| 2020 |  | 2019
Expected term in years | 6.07 - 6.09 |  | 5.99 - 6.25
Expected volatility | 71.42% |  | 74.84%
Expected risk-free interest rate | 0.80% - 1.65% |  | 2.45% - 2.62%
Expected dividend | 0% |  | 0%
Schedule of Restricted Stock Units Activity Under Plan | The following summarizes restricted stock unit activity under the 2018 Plan:
| Outstanding Restricted Stock Units |  |  |  |  |  |
| Restricted Stock Units |  | Weighted- Average Fair Value at Date of Grant per Share |  | Weighted Average Remaining Vesting Life |  | Aggregate Intrinsic Value
|  |  |  |  | (in years) |  | (in thousands)
Unvested December 31, 2019 | 1,941,155 |  |  | $ | 27.45 |  |  | 1.98 |  | $50,431
Granted | 885,267 |  |  | 18.55 |  |  | 2.44 |  |
Vested | (270,751) |  |  | 27.32 |  |  |  |  |
Forfeited | (21,000) |  |  | 28.19 |  |  |  |  |
Unvested March 31, 2020 | 2,534,671 |  |  | $ | 24.35 |  |  | 2.17 |  | $49,274
Vested and expected to vest, March 31, 2020 | 2,534,671 |  |  | $ | 24.35 |  |  | 2.17 |  | $49,274
Schedule of Stock-Based Compensation Expense | Total stock-based compensation related to stock options, restricted stock units, employee stock purchase plan and vesting of the founders’ common stock was as follows (in thousands):
| Three Months Ended March 31 |  |
| 2020 |  | 2019
Research and development | $ | 6,657 |  |  | $ | 2,732 |
General and administrative | 7,558 |  |  | 5,135 |
Total stock-based compensation | $ | 14,215 |  |  | $ | 7,867 |

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- Definition Tabular disclosure of the allocation of equity-based compensation costs to a given line item on the balance sheet and income statement for the period. This may include the reporting line for the costs and the amount capitalized and expensed. + 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 (h)(1) -URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SAB Topic 14.F) -URI http://asc.fasb.org/extlink&oid=115993241&loc=d3e301413-122809 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5047-113901 + Details
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- Definition Tabular disclosure for stock option plans. Includes, but is not limited to, outstanding awards at beginning and end of year, grants, exercises, forfeitures, and weighted-average grant date fair value. + 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 (d) -URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (e) -URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901 + Details
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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=109197908&loc=d3e5070-113901 + Details
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