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Stock-Based Compensation (Tables) | 6 Months Ended
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Jun. 30, 2020
Share-based Payment Arrangement [Abstract] |
Stock-Based Compensation Expense | The following stock-based compensation expense was recognized for the periods indicated (in thousands):
| Three Months Ended June 30, |  |  |  |  |  | Six Months Ended June 30, |  |
| 2020 |  | 2019 |  |  |  | 2020 |  | 2019
Research and development | $ | 2,589 |  |  | $ | 2,453 |  |  |  |  | $ | 5,023 |  |  | $ | 4,267 |
General and administrative | 2,506 |  |  | 2,431 |  |  |  |  | 4,564 |  |  | 4,416 |
Total stock-based compensation expense | $ | 5,095 |  |  | $ | 4,884 |  |  |  |  | $ | 9,587 |  |  | $ | 8,683 |
Valuation Assumptions Using the Black-Scholes Option-Pricing Model | The fair value of each option award is estimated on the date of grant using the Black-Scholes option-pricing model using the assumptions in the following table for options issued during the period indicated:
| Six Months Ended June 30, |  |
| 2020 |  | 2019
Expected dividend yield | 0% |  | 0%
Expected volatility | 67.3% - 108.8% |  | 73.7% - 75.3%
Risk-free interest rate | 0.5% - 1.8% |  | 1.9% - 2.6%
Expected term | 6.25 years |  | 6.25 years
Stock Option and Restricted Stock Unit Activity | The following table summarizes stock option activity during the six months ended June 30, 2020:
| Shares |  | Weighted- Average Exercise Price |  | Weighted-Average Remaining Contractual Term (Years) |  | Aggregate Intrinsic Value (in thousands)
Outstanding, December 31, 2019 | 6,706,994 |  |  | $ | 22.33 |  |  | 6.9 |  |
Granted | 1,547,627 |  |  | 12.67 |  |  |  |  |
Exercised | (329,592) |  |  | 7.31 |  |  |  |  |
Forfeited or expired | (650,489) |  |  | 20.90 |  |  |  |  |
Outstanding, June 30, 2020 | 7,274,540 |  |  | 21.07 |  |  | 7.2 |  | $ | 54,434 |
As of June 30, 2020: |  |  |  |  |  |  |
Exercisable | 4,134,978 |  |  | 23.32 |  |  | 5.9 |  | 23,276 |
Vested and expected to vest | 6,954,436 |  |  | 21.17 |  |  | 7.1 |  | 51,498 |
Schedule of RSU Activity | The following table summarizes RSU activity during the six months ended June 30, 2020:
| Shares |  | Weighted-Average Grant Date Fair Value
Outstanding, December 31, 2019 | 452,500 |  |  | $ | 15.32 |
Granted | 15,000 |  |  | 23.68 |
Exercised | — |  |  | — |
Forfeited | (29,100) |  |  | 15.32 |
Outstanding, June 30, 2020 | 438,400 |  |  | 15.61 |

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- Definition Tabular disclosure of allocation of amount expensed and capitalized for award under share-based payment arrangement to statement of income or comprehensive income and statement of financial position. Includes, but is not limited to, corresponding line item in financial statement. + 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=120381028&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=120381028&loc=d3e5047-113901 + 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=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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