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Equity Incentive Plans (Tables) | 12 Months Ended
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Dec. 31, 2019
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract] |
Summary of Fair Value of Stock Option Award Granted | The following assumptions were used to estimate the fair value of stock option awards granted to employees under the Company’s equity incentive plans and the shares purchasable under the ESPP during the periods presented:
|  | Year Ended December 31,
|  | 2019 |  | 2018 |  | 2017
Employee Stock Options |  |  |  |  |  |
Expected term (in years) |  | 4.6 - 6.1 |  | 5.3 - 6.1 |  | N/A
Risk-free interest rate |  | 1.38% - 2.58% |  | 2.65% - 2.96% |  | N/A
Volatility |  | 70.25% - 86.92% |  | 69.13% - 77.62% |  | N/A
Dividend yield |  | – |  | – |  | N/A
Employee Stock Purchase Plan |  |  |  |  |  |
Expected term (in years) |  | 0.49 - 1.99 |  | N/A |  | N/A
Risk-free interest rate |  | 1.46% - 1.87% |  | N/A |  | N/A
Volatility |  | 72.08% - 78.70% |  | N/A |  | N/A
Dividend yield |  | – |  | N/A |  | N/A
Summary of Stock Option Activity | Stock Options The following table summarizes stock option activity for the years ended December 31, 2019 and 2018:
|  | Shares Subject to Options Outstanding |  |  | Weighted- Average |  |  |  |  |
|  |  |  |  |  | Weighted- |  |  | Remaining |  |  |  |  |
|  |  |  |  |  | Average Exercise |  |  | Contractual Life |  |  | Aggregate |
|  | Shares |  |  | Price |  |  | (Years) |  |  | Intrinsic Value |
|  |  |  |  |  |  |  |  |  |  |  |  |  | (in thousands) |
Outstanding as of December 31, 2017 |  |  | — |  |  |  | — |  |  |  | — |  |  | $ | — |
Options granted |  |  | 5,143,551 |  |  | $ | 7.49 |  |  |  |  |  |  |  |  |
Option exercised |  |  | — |  |  | $ | — |  |  |  |  |  |  |  |  |
Options forfeited/cancelled |  |  | (36,222 | ) |  | $ | 4.85 |  |  |  |  |  |  |  |  |
Outstanding as of December 31, 2018 |  |  | 5,107,329 |  |  | $ | 7.51 |  |  |  | 9.7 |  |  | $ | 16,343 |
Options granted |  |  | 4,194,624 |  |  | $ | 20.11 |  |  |  |  |  |  |  |  |
Option exercised |  |  | (419,593 | ) |  | $ | 4.87 |  |  |  |  |  |  |  |  |
Options forfeited/cancelled |  |  | (344,300 | ) |  | $ | 11.64 |  |  |  |  |  |  |  |  |
Outstanding as of December 31, 2019 |  |  | 8,538,060 |  |  | $ | 13.67 |  |  |  | 9.0 |  |  | $ | 35,385 |
Options vested and exercisable as of December 31, 2019 |  |  | 1,215,471 |  |  | $ | 7.25 |  |  |  | 8.7 |  |  | $ | 10,183 |
Summary of Restricted Stock Activity | The summary of the Company’s restricted stock activity during the years ended December 31, 2019, 2018 and 2017 is as follows:
|  | Number of |  |  | Weighted- |
|  | Restricted |  |  | Average |
|  | Stock Units |  |  | Grant Date |
|  | Outstanding |  |  | Fair Value |
Nonvested at December 31, 2016 |  |  | — |  |  | $ | — |
Granted |  |  | 1,305,421 |  |  |  | 0.09 |
Nonvested at December 31, 2017 |  |  | 1,305,421 |  |  | $ | 0.09 |
Granted |  |  | 8,673,584 |  |  |  | 5.53 |
Vested |  |  | (2,369,696 | ) |  |  | 7.58 |
Forfeited |  |  | (127,277 | ) |  |  | 0.09 |
Nonvested at December 31, 2018 |  |  | 7,482,032 |  |  | $ | 4.01 |
Granted |  |  | — |  |  |  | — |
Vested |  |  | (2,833,506 | ) |  |  | 4.05 |
Forfeited |  |  | — |  |  |  | — |
Nonvested at December 31, 2019 |  |  | 4,648,526 |  |  | $ | 3.98 |
Stock-Based Compensation Expense Reported in Condensed Consolidated Statements of Operations | Stock-based compensation expense has been reported in the Company’s consolidated statements of operations as follows (in thousands):
|  | Year Ended |
|  | December 31, |
|  | 2019 |  |  | 2018 |  |  | 2017 |
Research and development |  | $ | 10,227 |  |  | $ | 679 |  |  | $ | 17 |
General and administrative |  |  | 10,592 |  |  |  | 30,268 |  |  |  | 15 |
Total stock-based compensation |  | $ | 20,819 |  |  | $ | 30,947 |  |  | $ | 32 |

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- Definition Tabular disclosure of the significant assumptions used during the year to estimate the fair value of employee stock purchase plans, including, but not limited to: (a) expected term, (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=116856206&loc=d3e5070-113901 + Details
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