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Stock Based Compensation (Tables) | 9 Months Ended
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Sep. 30, 2018
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract] |
Summary of Option Activity Under Company's Equity Incentive Plans | The following table summarizes stock option activity under the Company’s equity incentive plans and related information:
|  | Number of Shares Underlying Outstanding Options |  |  | Weighted- Average Exercise Price |  |  | Weighted-Average Remaining Contractual Term (Years) |
Outstanding — December 31, 2017 |  |  | 821,741 |  |  | $ | 13.69 |  |  |  | 9.17 |
Options granted |  |  | 564,050 |  |  | $ | 13.43 |  |  |  |  |
Options exercised |  |  | (7,707 | ) |  | $ | 4.63 |  |  |  |  |
Options expired |  |  | (15,667 | ) |  | $ | 158.94 |  |  |  |  |
Options canceled/forfeited |  |  | (43,315 | ) |  | $ | 7.76 |  |  |  |  |
Outstanding — September 30, 2018 |  |  | 1,319,102 |  |  | $ | 12.10 |  |  |  | 8.92 |
Exercisable — September 30, 2018 |  |  | 344,273 |  |  | $ | 19.48 |  |  |  | 8.06 |
Summary of Weighted Average Valuation Assumptions Used to Estimate Fair Value of Employee Stock Options | The fair value of employee stock options was estimated using the following weighted-average assumptions for the nine months ended September 30, 2018 and 2017:
|  | Nine Months Ended September 30, |
|  | 2018 |  |  | 2017 |
Employee Stock Options: |  |  |  |  |  |  |  |
Risk-free interest rate |  |  | 2.66 | % |  |  | 2.02 | %
Expected term (in years) |  |  | 5.97 |  |  |  | 5.99 |
Dividend yield |  |  | — |  |  |  | — |
Volatility |  |  | 93.60 | % |  |  | 110.42 | %
Weighted-average fair value of stock options granted |  | $ | 10.28 |  |  | $ | 3.79 |
Summary of Stock-Based Compensation Recognized | Total stock-based compensation recognized was as follows (in thousands):
|  | Three Months Ended September 30, |  |  | Nine Months Ended September 30, |
|  | 2018 |  |  | 2017 |  |  | 2018 |  |  | 2017 |
Research and development |  | $ | 161 |  |  | $ | 62 |  |  | $ | 413 |  |  | $ | 97 |
General and administrative(1) |  |  | 541 |  |  |  | 263 |  |  |  | 1,445 |  |  |  | 459 |
Total stock-based compensation |  | $ | 702 |  |  | $ | 325 |  |  | $ | 1,858 |  |  | $ | 556 |
| (1) | 2018 includes $0.1 million in modification stock-based compensation expense related to a Board member’s departure.

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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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