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Stock-based Compensation (Tables) | 6 Months Ended
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Jun. 30, 2018
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract] |
Weighted Average Assumptions | The fair value of the stock options granted during the three and six months ended June 30, 2018 and 2017, was estimated based on the following weighted average assumptions:
| Three Months Ended June 30, |  |  | Six Months Ended June 30, |
| 2018 |  |  | 2017 |  |  | 2018 |  |  | 2017 |
Dividend yield |  | 0 | % |  |  | 0 | % |  |  | 0 | % |  |  | 0 | %
Expected volatility |  | 102 | % |  |  | 100 | % |  |  | 101 | % |  |  | 100 | %
Risk-free interest rate |  | 2.98 | % |  |  | 1.78 | % |  |  | 2.71 | % |  |  | 1.78 | %
Expected term | 5.55 years |  |  | 5.55 years |  |  | 5.60 years |  |  | 5.55 years |
Stock Option Activity | Stock option activity for the six months ended June 30, 2018, was as follows:
| Number of shares |  |  | Weighted Average Exercise Price Per Share |  |  | Weighted Average Contractual Term (in years) |  |  | Aggregate Intrinsic Value (in thousands) |
Outstanding at beginning of year |  | 1,622,065 |  |  | $ | 24.54 |  |  |  |  |  |  |  |  |
Granted |  | 233,208 |  |  |  | 28.98 |  |  |  |  |  |  |  |  |
Exercised |  | (44,400 | ) |  |  | 5.49 |  |  |  |  |  |  |  |  |
Forfeited, expired or cancelled |  | (202,941 | ) |  |  | 44.07 |  |  |  |  |  |  |  |  |
Outstanding at end of period |  | 1,607,932 |  |  | $ | 23.25 |  |  |  | 8.2 |  |  | $ | 16,650 |
Exercisable at end of period |  | 922,333 |  |  | $ | 28.96 |  |  |  | 7.5 |  |  | $ | 10,411 |
RSU Activity | RSU activity for the six months ended June 30, 2018, is summarized below:
|  |  |  |  | Weighted- |  |
Restricted Stock Units: | Number of |  |  | Average Grant- |  |
| Shares |  |  | Date Fair Value |  |
Unvested balance at January 1, 2018 |  | 18,480 |  |  | $ | 18.00 |  |
Granted |  | — |  |  |  | — |  |
Vested |  | (17,614 | ) |  |  | 18.54 |  |
Forfeited |  | — |  |  |  | — |  |
Unvested balance at June 30, 2018 |  | 866 |  |  | $ | 7.02 |  |
Stock-based Compensation Expense | The following table shows total stock-based compensation expense for stock options, RSUs and ESPP in the condensed consolidated statements of operations and comprehensive (loss) income (in thousands):
| Three Months Ended June 30, |  |  | Six Months Ended June 30, |
| 2018 |  |  | 2017 |  |  | 2018 |  |  | 2017 |
Research and development | $ | 95 |  |  | $ | 231 |  |  | $ | 199 |  |  | $ | 672 |
General and administrative |  | 675 |  |  |  | 1,623 |  |  |  | 1,987 |  |  |  | 2,183 |
Total stock-based compensation expense | $ | 770 |  |  | $ | 1,854 |  |  | $ | 2,186 |  |  | $ | 2,855 |

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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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