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Stock Incentive Plans - Stock Option Plan Activity (Details) - USD ($) $ / shares in Units, $ in Thousands | 12 Months Ended
--- | ---
Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Stock options |  |  |
Outstanding at beginning of year (in shares) | 2,270,169 |  |
Granted (in shares) | 890,740 |  |
Exercised (in shares) | (22,879) |  |
Cancelled or forfeited (in shares) | (292,404) |  |
Outstanding at end of year (in shares) | 2,845,626 | 2,270,169 |
Vested and Exercisable to vest at end of year (in shares) | 1,265,411 |  |
Weighted-Average Exercise Price |  |  |
Outstanding at beginning of year (in dollars per share) | $ 6.11 |  |
Granted (in dollars per share) | 1.26 |  |
Exercised (in dollars per share) | 1.01 |  |
Cancelled or forfeited (in dollars per share) | 7.13 |  |
Outstanding at end of year (in dollars per share) | 4.53 | $ 6.11 |
Vested and Exercisable at end of year (in dollars per share) | $ 5.91 |  |
Weighted Average Remaining Contractual Term (years) |  |  |
Outstanding at end of year | 7 years 9 months | 8 years 1 month 21 days |
Vested and Exercisable at end of year | 6 years 6 months 11 days |  |
Aggregate Intrinsic Value |  |  |
Outstanding at the beginning of the year | $ 613 |  |
Outstanding at the end of the year | $ 216 | $ 613 |
Weighted average of the assumptions used to compute employee stock based compensation |  |  |
Expected term (in years) |  | 6 years 3 months |
Expected dividend yield (as a percent) | 0.00% | 0.00% | 0.00%
Stock options |  |  |
Unrecognized stock-based compensation expense |  |  |
Intrinsic value of options exercised | $ 40 | $ 500 | $ 300
Adjusted fair value of options vested | $ 2,200 | $ 2,300 | $ 1,400
Weighted average grant date fair value of options granted (in dollars per share) | $ 0.85 | $ 2.89 | $ 7.68
Unrecognized compensation expense related to unvested stock option awards | $ 3,200 |  |
Weighted-average amortization period over which cost is expected to be recognized | 2 years |  |
Minimum |  |  |
Weighted average of the assumptions used to compute employee stock based compensation |  |  |
Weighted-average expected volatility (as a percent) | 76.10% | 72.00% | 73.60%
Expected term (in years) | 6 years 3 months |  | 6 years 2 months 1 day
Risk-free interest rate (as a percent) | 2.00% | 1.24% | 0.92%
Maximum |  |  |
Weighted average of the assumptions used to compute employee stock based compensation |  |  |
Weighted-average expected volatility (as a percent) | 81.90% | 73.90% | 86.80%
Expected term (in years) | 9 years 9 months 7 days |  | 10 years
Risk-free interest rate (as a percent) | 2.38% | 2.00% | 2.45%

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- Definition Weighted average period over which unrecognized compensation is expected to be recognized for equity-based compensation plans, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + 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 (i) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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- Definition The estimated dividend rate (a percentage of the share price) to be paid (expected dividends) to holders of the underlying shares over the option's term. + 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)(iii) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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- Definition The estimated measure of the percentage by which a share price is expected to fluctuate during a period. Volatility also may be defined as a probability-weighted measure of the dispersion of returns about the mean. The volatility of a share price is the standard deviation of the continuously compounded rates of return on the share over a specified period. That is the same as the standard deviation of the differences in the natural logarithms of the stock prices plus dividends, if any, over the period. + 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)(ii) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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- Definition The risk-free interest rate assumption that is used in valuing an option on its own shares. + 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)(iv) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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- Definition For presentations that combine terminations, the number of shares under options that were cancelled during the reporting period as a result of occurrence of a terminating event specified in contractual agreements pertaining to the stock option plan or that expired. + 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 (c)(1)(iv)(3)-(4) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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- Definition Weighted average price of options that were either forfeited or expired. + 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 (c)(1)(iv)(3)-(4) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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- Definition Amount by which the current fair value of the underlying stock exceeds the exercise price of options outstanding. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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- Definition Weighted average price at which grantees can acquire the shares reserved for issuance under the stock option plan. + 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 (c)(1)(i) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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- Definition Number of fully vested and expected to vest options outstanding that can be converted into shares under option plan. Includes, but is not limited to, unvested options for which requisite service period has not been rendered but that are expected to vest based on achievement of performance condition, if forfeitures are recognized when they occur. + 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 (e)(1) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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