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Stock-based Compensation (Tables) | 3 Months Ended
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Mar. 31, 2021
Stock-based Compensation |
Summary of performance share units activity | ​
​ | ​ | ​ | ​ | ​ | ​
​ | ​ | ​ | ​ | Weighted-Average
​ | ​ | Shares | ​ | Grant Date Fair Value
Outstanding at December 31, 2020 | ​ | 283,223 | ​ | $ | 24.26
Granted | ​ | 213,180 | ​ | ​ | 35.12
Vested | ​ | (60,990) | ​ | ​ | 22.45
Forfeited | ​ | — | ​ | ​ | —
Performance adjustment | ​ | (12,609) | ​ | ​ | 21.80
Outstanding at March 31, 2021 | ​ | 422,804 | ​ | $ | 30.07
Summary of restricted stock units activity | ​
​ | ​ | ​ | ​ | ​ | ​
​ | ​ | ​ | ​ | Weighted-Average
​ | ​ | Shares | ​ | Grant Date Fair Value
Outstanding at December 31, 2020 | ​ | 1,242,387 | ​ | $ | 19.42
Granted | ​ | 1,163,664 | ​ | ​ | 24.43
Vested | ​ | (352,548) | ​ | ​ | 19.42
Forfeited | ​ | (14,968) | ​ | ​ | 21.35
Outstanding at March 31, 2021 | ​ | 2,038,535 | ​ | $ | 22.27
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, 2020 |  | 3,860,481 | ​ | $ | 17.78 |  | 7.2 | ​ | $ | 13,011
Granted |  | 63,000 | ​ | ​ | 19.97 | ​ | ​ | ​ | ​ | ​
Exercised |  | (289,164) | ​ | ​ | 14.18 | ​ | ​ | ​ | ​ | ​
Cancelled |  | (25,491) | ​ | ​ | 19.30 | ​ | ​ | ​ | ​ | ​
Outstanding at March 31, 2021 |  | 3,608,826 | ​ | $ | 18.09 |  | 7.0 | ​ | $ | 20,895
Exercisable at March 31, 2021 |  | 2,353,069 | ​ | $ | 17.57 |  | 6.3 | ​ | $ | 14,949
Schedule of fair value assumption using Black-Scholes option-pricing model | ​
​ | ​ | ​ | ​ | ​ | ​
​ | Three months ended March 31,
​ | 2021 | ​ | 2020
Risk-free interest rate | 0.5 | % | ​ | 1.5 | %
Volatility | 68.3 | % | ​ | 65.5 | %
Expected term (years) | 6.0 | ​ | ​ | 6.1 | ​
Expected dividend yield | — | % | ​ | — | %
Summary of stock-based compensation | ​
​ | ​ | ​ | ​ | ​ | ​
​ | Three months ended March 31,
​ | 2021 | ​ | 2020
Research and development | $ | 1,217 |  | $ | 770
Selling, general and administrative |  | 5,662 | ​ |  | 4,181
Total stock-based compensation expense | $ | 6,879 | ​ | $ | 4,951

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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=120381028&loc=d3e5070-113901 + Details
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