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Stockholders' Equity (Tables) | 12 Months Ended
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Dec. 31, 2017
Stockholders' Equity |
Summary of deferred compensation amounts | The following table summarizes the deferred compensation amounts under the Deferral Plan as of December 31, 2017 and 2016, respectively (in thousands):
|  | Year ended December 31, |
|  | 2017 |  | 2016 |
Deferred compensation investment |  | $ | 2,248 |  | $ | 1,469 |
Deferred compensation liability |  | $ | 2,258 |  | $ | 1,479 |
Investment income |  | $ | 66 |  | $ | 34 |
Unrealized gain |  | $ | 92 |  | $ | 32 |
Schedule of fair value weighted-average assumptions |
|  | Years Ended December 31, |
|  | 2017 |  | 2016 |  | 2015 |
Expected stock price volatility |  |  | 82.8 | % |  | 81.3 | % |  | 75.9 | %
Risk free interest rate |  |  | 2.0 | % |  | 1.5 | % |  | 1.7 | %
Expected life of options (years) |  |  | 6.18 |  |  | 6.25 |  |  | 6.25 |
Expected annual dividend per share |  | $ | 0.00 |  | $ | 0.00 |  | $ | 0.00 |
Summary of stock options outstanding |
|  | Number of Shares |  | Weighted Average Exercise Price |  | Weighted Average Remaining Contractual Life |  | Aggregate Intrinsic Value |
|  | (in thousands) |  |  |  |  |  | (in millions) |
Options outstanding, December 31, 2014 |  |  | 10,020.7 |  | $ | 5.02 |  |  |  |  |  |
Granted |  |  | 3,917.2 |  | $ | 11.61 |  |  |  |  |  |
Exercised |  |  | (2,070.3 | ) | $ | 5.43 |  |  |  |  |  |
Forfeited |  |  | (138.4 | ) | $ | 7.76 |  |  |  |  |  |
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
Options outstanding, December 31, 2015 |  |  | 11,729.2 |  | $ | 7.11 |  |  |  |  |  |
Granted |  |  | 5,114.1 |  | $ | 7.67 |  |  |  |  |  |
Exercised |  |  | (723.1 | ) | $ | 4.20 |  |  |  |  |  |
Forfeited |  |  | (622.7 | ) | $ | 8.62 |  |  |  |  |  |
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
Options outstanding, December 31, 2016 |  |  | 15,497.5 |  | $ | 7.37 |  |  |  |  |  |
Granted |  |  | 3,695.3 |  | $ | 7.17 |  |  |  |  |  |
Exercised |  |  | (2,878.7 | ) | $ | 5.67 |  |  |  |  |  |
Forfeited |  |  | (1,133.0 | ) | $ | 9.55 |  |  |  |  |  |
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
Options outstanding, December 31, 2017 |  |  | 15,181.1 |  | $ | 7.48 |  | 7.2 years |  | $ | 105.8 |
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
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Vested and unvested expected to vest, December 31, 2017 |  |  | 14,398.5 |  | $ | 7.45 |  | 7.1 years |  | $ | 100.9 |
Exercisable at December 31, 2017 |  |  | 7,985.6 |  | $ | 7.05 |  | 6.0 years |  | $ | 59.1 |
Summary of non-vested Restricted Stock Units activity |
|  | Number of Share |  | Weighted Average Grant Date Fair Value |  | Weighted Average Remaining Years |  | Aggregate Intrinsic Value |
|  | (in thousands) |  |  |  |  |  | (in millions) |
Non-vested units as of December 31, 2015 |  |  | 478.5 |  | $ | 10.38 |  |  |  |  |  |  |
Granted |  |  | 582.7 |  | $ | 6.21 |  |  |  |  |  |  |
Vested |  |  | (281.9 | ) | $ | 8.73 |  |  |  |  |  |  |
Forfeited |  |  | (34.9 | ) | $ | 7.71 |  |  |  |  |  |  |
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
Non-vested units as of December 31, 2016 |  |  | 744.4 |  | $ | 7.86 |  |  |  |  |  |  |
Granted |  |  | 2,348.7 |  | $ | 5.69 |  |  |  |  |  |  |
Vested |  |  | (318.2 | ) | $ | 9.23 |  |  |  |  |  |  |
Forfeited |  |  | (199.8 | ) | $ | 6.24 |  |  |  |  |  |  |
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
Non-vested units as of December 31, 2017 |  |  | 2,575.1 |  | $ | 5.85 |  |  | 2.47 |  | $ | 37.1 |
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
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Summary of the equity-based compensation expense recognized in the statements of operations | The following table summarizes the equity-based compensation expense recognized in the statements of operations (in thousands):
|  | Years Ended December 31, |
|  | 2017 |  | 2016 |  | 2015 |
Equity compensation expense recognized in: |  |  |  |  |  |  |  |  |  |
Research and development expense |  | $ | 10,328 |  | $ | 8,071 |  | $ | 4,600 |
Selling, general and administrative expense |  |  | 12,773 |  |  | 9,433 |  |  | 5,372 |
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
Total equity compensation expense |  | $ | 23,101 |  | $ | 17,504 |  | $ | 9,972 |
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
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