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Stock-Based Compensation (Tables) | 9 Months Ended
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Sep. 30, 2019
Disclosure of Compensation Related Costs, Share-based Payments [Abstract] |
Stock-Based Compensation Expense | ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ |  | Three Months Ended |  | Nine Months Ended
​ | ​ | September 30, | ​ | September 30,
​ |  | 2019 |  | 2018 |  | 2019 |  | 2018
​ | ​ | (In thousands) | ​ | (In thousands)
Research and development | ​ | $ | 1,558 | ​ | $ | 1,334 | ​ | $ | 4,698 | ​ | $ | 3,720
Selling, general and administrative | ​ |  | 1,938 | ​ |  | 1,595 | ​ |  | 5,770 | ​ |  | 5,176
Total | ​ | $ | 3,496 | ​ | $ | 2,929 | ​ | $ | 10,468 | ​ | $ | 8,896
​
Schedule of Share-based Payment Award, Stock Options, Valuation Assumptions | ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ |  | Three Months Ended | ​ | ​ | Nine Months Ended | ​
​ | ​ | September 30, | ​ | ​ | September 30, | ​
​ |  | 2019 |  | 2018 |  | ​ | 2019 |  | 2018 | ​
Estimated weighted-average fair value | ​ | $ | 11.81 | ​ | $ | 16.30 | ​ | ​ | $ | 9.91 | ​ | $ | 10.22 | ​
Weighted-average assumptions: | ​ |  |  | ​ |  |  | ​ | ​ |  |  | ​ |  |  | ​
Expected volatility | ​ |  | 79 | % |  | 78 | % | ​ |  | 81 | % |  | 77 | %
Expected term, in years | ​ |  | 6.1 | ​ |  | 6.1 | ​ | ​ |  | 6.0 | ​ |  | 6.0 | ​
Risk-free interest rate | ​ |  | 2.10 | % |  | 2.77 | % | ​ |  | 2.44 | % |  | 2.66 | %
Expected dividend yield | ​ |  | — | % |  | — | % | ​ |  | — | % |  | — | %
Stock Option Activity and Related Information | ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ |  | ​ |  | Weighted- |  | ​ |  | ​ | ​
​ | ​ | ​ | ​ | Average | ​ | ​ | ​ | Aggregate
​ | ​ | ​ | ​ | Exercise | ​ | Remaining | ​ | Intrinsic
​ | ​ | Options | ​ | Price per | ​ | Contractual Life | ​ | Value
​ | ​ | Outstanding | ​ | Share | ​ | (In years) | ​ | (In thousands)
Balance at December 31, 2018 |  | 10,313,138 | ​ | $ | 11.22 |  |  |  | ​ |
Granted |  | 1,956,205 | ​ |  | 14.17 |  |  |  | ​ |
Exercised |  | (513,790) | ​ |  | 9.80 |  |  |  | ​ |
Forfeited |  | (260,831) | ​ |  | 16.07 |  |  |  | ​ |
Balance at September 30, 2019 |  | 11,494,722 | ​ | $ | 11.67 |  | 6.25 | ​ | $ | 56,610
Vested and expected to vest at September 30, 2019 |  | 11,126,906 | ​ | $ | 11.59 |  | 6.17 | ​ | $ | 55,668
Exercisable at September 30, 2019 |  | 8,117,842 | ​ | $ | 10.68 |  | 5.20 | ​ | $ | 47,725
​

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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=116856206&loc=d3e5070-113901 + Details
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