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Stock-Based Compensation (Tables) | 9 Months Ended
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Sep. 30, 2019
Disclosure of Compensation Related Costs, Share-based Payments [Abstract] |
Stock-Based Compensation Expense | The following stock-based compensation expense was recognized for the periods indicated (in thousands):
| Three Months Ended September 30, |  |  |  | Nine Months Ended September 30, |  |
| 2019 |  | 2018 |  | 2019 |  | 2018
Research and development | $ | 2,765 |  |  | $ | 2,440 |  |  | $ | 7,032 |  |  | $ | 6,347 |
General and administrative | 2,633 |  |  | 2,133 |  |  | 7,049 |  |  | 5,821 |
Total stock-based compensation expense | $ | 5,398 |  |  | $ | 4,573 |  |  | $ | 14,081 |  |  | $ | 12,168 |
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:
| Nine Months Ended September 30, |  |
| 2019 |  | 2018
Expected dividend yield | 0% |  |  | 0% |
Expected volatility | 73.7% - 76.6% |  | 67.8% - 72.2%
Risk-free interest rate | 1.4% - 2.6% |  | 2.4% - 3.1%
Expected term | 6.25 years |  | 6.25 years
Stock Option and Restricted Stock Unit Activity | The following table summarizes stock option activity during the nine months ended September 30, 2019:
| Shares |  | Weighted- Average Exercise Price |  | Weighted-Average Remaining Contractual Term (Years) |  | Aggregate Intrinsic Value (in thousands)
Outstanding, December 31, 2018 | 5,273,964 |  |  | $ | 22.23 |  |  | 6.8 |  |
Granted | 2,384,770 |  |  | 16.87 |  |  |  |  |
Exercised | (210,261) |  |  | 3.11 |  |  |  |  |
Forfeited or expired | (199,944) |  |  | 23.44 |  |  |  |  |
Outstanding, September 30, 2019 | 7,248,529 |  |  | 20.98 |  |  | 7.3 |  | $ | 11,733 |
As of September 30, 2019: |  |  |  |  |  |  |
Exercisable | 4,074,460 |  |  | 22.13 |  |  | 5.9 |  | 5,849 |
Vested and expected to vest | 6,936,223 |  |  | 20.98 |  |  | 7.2 |  | 11,367 |

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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 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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