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Stock-Based Compensation (Tables) | 3 Months Ended
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Mar. 31, 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 March 31, |  |  |  |  |  |
| 2019 |  | 2018 |  |  |  |
Research and development | $ | 1,814 |  | $ | 1,700 |  |  |  |
General and administrative | 1,985 |  | 1,732 |  |  |  |
Total stock-based compensation expense | $ | 3,799 |  | $ | 3,432 |  |  |  |
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:
| Three Months Ended March 31, |  |
| 2019 |  | 2018
Expected dividend yield | —% |  |  | —% |
Expected volatility | 73.7% - 74.4% |  | 68% - 71%
Risk-free interest rate | 2.3% - 2.6% |  | 2.4% - 2.8%
Expected term | 6.25 years |  | 6.25 years
Stock Option and Restricted Stock Unit Activity | The following table summarizes stock option activity during the three months ended March 31, 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 | 1,628,022 |  | 21.82 |  |  |  |
Exercised | (126,707) |  | 2.84 |  |  |  |
Forfeited or expired | (55,032) |  | 21.02 |  |  |  |
Outstanding, March 31, 2019 | 6,720,247 |  | 22.48 |  | 7.4 |  | $ | 10,712
As of March 31, 2019: |  |  |  |  |  |  |
Exercisable | 3,593,089 |  | 21.44 |  | 5.9 |  | 10,670
Vested and expected to vest | 6,407,781 |  | 22.41 |  | 7.4 |  | 10,707

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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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