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Stock Incentive Plans (Tables) | 12 Months Ended
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Dec. 31, 2017
Stock Incentive Plans |
Summary of stock option activity |
|  | Shares |  | Weighted- Average Exercise Price |  | Weighted Average Remaining Contractual Term (years) |  | Aggregate Intrinsic Value (in thousands) |
Outstanding at December 31, 2016 |  |  | 2,270,169 |  | $ | 6.11 |  |  | 8.14 |  | $ | 613 |
Granted |  |  | 890,740 |  | $ | 1.26 |  |  |  |  |  |  |
Exercised |  |  | (22,879 | ) | $ | 1.01 |  |  |  |  |  |  |
Cancelled or forfeited |  |  | (292,404 | ) | $ | 7.13 |  |  |  |  |  |  |
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
Outstanding at December 31, 2017 |  |  | 2,845,626 |  | $ | 4.53 |  |  | 7.75 |  | $ | 216 |
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
Vested and Exercisable at December 31, 2017 |  |  | 1,265,411 |  | $ | 5.91 |  |  | 6.53 |  | $ | — |
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
Schedule of assumptions made based on the Black-Scholes option pricing model |
|  | Year Ended December 31, |
|  | 2017 |  | 2016 |  | 2015 |
Weighted-average expected volatility |  |  | 76.1 - 81.9 | % |  | 72.0 - 73.9 | % |  | 73.6 - 86.8 | %
Expected term (in years) |  |  | 6.25 - 9.77 |  |  | 6.25 |  |  | 6.17 - 10.00 |
Risk-free interest rate |  |  | 2.00 - 2.38 | % |  | 1.24 - 2.00 | % |  | 0.92 - 2.45 | %
Expected dividend yield |  |  | 0 | % |  | 0 | % |  | 0 | %

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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 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://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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