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Equity (Tables) | 3 Months Ended
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Mar. 31, 2018
Equity [Abstract] |
Summary of Stock Option Activity | A summary of the stock option activity under the Company’s plans for the three months ended March 31, 2018 is as follows:
|  | Number of shares |  | Weighted average exercise price
Options outstanding at December 31, 2017 |  | 6,342,526 |  |  | $ | 6.51 |
Options granted |  | 1,720,500 |  |  | $ | 8.06 |
Less: |  |  |  |
Options exercised |  | 72,500 |  |  | $ | 4.14 |
Options canceled or expired |  | 12,500 |  |  | $ | 7.52 |
Options outstanding at March 31, 2018 |  | 7,978,026 |  |  | $ | 6.86 |
Exercisable at March 31, 2018 |  | 2,250,840 |  |  | $ | 5.63 |
Fair Value of Each Stock Option Award Granted | The fair value of each stock option award granted during the three months ended March 31, 2018 was estimated as of the grant date using a trinomial lattice model with the following weighted average assumptions:
| Three Months Ended March 31, 2018
Expected term (in years) | 2.0 - 4.0
Risk-free interest rate (%) | 2.4%
Expected volatility (%) | 35.6% - 45.5%
Dividend yield (%) | —
Weighted average fair value/share at grant date | $2.78
Summary of Stock-Based Compensation Expense Recognized for Stock Options and Restricted Stock Included in Consolidated Statements of Operations | Stock based compensation expense recognized for stock options and restricted stock and included in the consolidated statements of operations was allocated as follows (in thousands):
|  | Three Months Ended March 31,
|  | 2018 |  | 2017
Research and development expense |  | $ | 18 |  |  | $ | 43 |
General and administrative expense |  | 1,606 |  |  | 1,087 |
Total stock based compensation expense |  | $ | 1,624 |  |  | $ | 1,130 |

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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://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (h)(1) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5047-113901 Reference 3: http://www.xbrl.org/2003/role/presentationRef -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=27013229&loc=d3e301413-122809 + 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://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (e) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=96867065&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 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://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (f)(2) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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