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Stock-based Compensation (Tables) | 6 Months Ended
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Jun. 30, 2019
Summary of stock-based compensation | ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | Three months ended June 30, | ​ | Six months ended June 30,
​ | 2019 | ​ | 2018 | ​ | 2019 | ​ | 2018
Research and development expenses | $ | 514 |  | $ | 380 | ​ | $ | 1,081 |  | $ | 703
Selling, general and administrative expenses |  | 3,648 | ​ |  | 3,146 | ​ |  | 7,344 | ​ |  | 5,551
Total stock-based compensation expense | $ | 4,162 | ​ | $ | 3,526 | ​ | $ | 8,425 | ​ | $ | 6,254
Summary of restricted stock units activity | ​
​ | ​ | ​ | ​ | ​ | ​
​ | ​ | ​ | ​ | Weighted-Average
​ | ​ | Shares | ​ | Grant Date Fair Value
Outstanding at December 31, 2018 | ​ | 514,603 | ​ | $ | 20.67
Granted | ​ | 590,205 | ​ | ​ | 15.78
Vested | ​ | (127,787) | ​ | ​ | 21.54
Forfeited | ​ | (84,784) | ​ | ​ | 18.26
Outstanding at June 30, 2019 | ​ | 892,237 | ​ | $ | 17.54
Summary of stock option activity | ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ |  | ​ |  | ​ | ​ |  | Weighted- |  | ​ | ​
​ | ​ | ​ |  | ​ | Weighted- |  | Average | ​ | ​ | ​
​ | ​ | ​ |  | ​ | Average |  | Remaining | ​ | ​ | Aggregate
​ | ​ | ​ | ​ | ​ | Exercise Price |  | Contractual | ​ | ​ | Intrinsic
​ |  | Shares |  | ​ | per Share |  | Term (in years) |  | ​ | Value
Outstanding at December 31, 2018 |  | 3,585,856 | ​ | $ | 16.20 |  | 8.0 | ​ | $ | 11,170
Granted |  | 897,312 | ​ |  | 15.67 | ​ | ​ | ​ | ​ | ​
Exercised |  | (26,911) | ​ | ​ | 11.30 | ​ | ​ | ​ | ​ | ​
Cancelled |  | (266,141) | ​ |  | 17.61 | ​ | ​ | ​ | ​ | ​
Outstanding at June 30, 2019 |  | 4,190,116 | ​ | $ | 16.03 |  | 7.7 | ​ | $ | 5,248
Exercisable at June 30, 2019 |  | 2,113,346 | ​ | $ | 14.45 |  | 6.6 | ​ | $ | 4,285
Schedule of fair value assumption using Black-Scholes option-pricing model | ​
​ | ​ | ​ | ​ | ​ | ​
​ | Six months ended June 30,
​ | 2019 | ​ | 2018
Risk-free interest rate | 2.6 | % | ​ | 2.6 | %
Volatility | 63.3 | % | ​ | 65.0 | %
Expected term (years) | 6.06 | ​ | ​ | 6.11 | ​
Expected dividend yield | — | % | ​ | — | %
Performance share units |
Summary of stock option activity | ​
​ | ​ | ​ | ​ | ​ | ​
​ | ​ | ​ | ​ | Weighted-Average
​ | ​ | Shares | ​ | Grant Date Fair Value
Outstanding at December 31, 2018 | ​ | — | ​ | $ | —
Granted | ​ | 99,400 | ​ | ​ | 15.90
Outstanding at June 30, 2019 | ​ | 99,400 | ​ | $ | 15.90

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- Definition Tabular disclosure of allocation of amount expensed and capitalized for award under share-based payment arrangement to statement of income or comprehensive income and statement of financial position. Includes, but is not limited to, corresponding line item in financial statement. + 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 (h)(1) -URI http://asc.fasb.org/extlink&oid=116856206&loc=d3e5070-113901 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SAB Topic 14.F) -URI http://asc.fasb.org/extlink&oid=115993241&loc=d3e301413-122809 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=116856206&loc=d3e5047-113901 + Details
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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=116856206&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=116856206&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=116856206&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=116856206&loc=d3e5070-113901 + Details
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