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Stock-based Compensation | 3 Months Ended
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Mar. 31, 2021
Stock-based Compensation |
Stock-based Compensation | 13. Stock-based Compensation ​ Performance Share Units, Restricted Stock Units and Stock Options ​ Performance Share Units ​ The Company periodically grants PSUs to certain members of the Company's senior management team. PSUs vest subject to the satisfaction of annual and cumulative performance and/or market conditions established by the Compensation Committee. ​ A summary of the Company’s PSU activity for the three months ended March 31, 2021 and related information is as follows: ​
​ | ​ | ​ | ​ | ​ | ​
​ | ​ | ​ | ​ | 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
​ The number of PSUs granted represents the target number of shares of common stock that may be earned, however, the actual number of shares earned may vary based on the satisfaction of performance criteria. The weighted-average grant date fair value of PSUs granted for the three months ended March 31, 2021 and 2020 was $35.12 and $28.49, respectively. ​ Restricted Stock Units ​ The Company granted RSUs to employees for the three months ended March 31, 2021. The Company’s RSUs generally vest ratably over a four-year period of service. A summary of the Company’s RSU activity for the three months ended March 31, 2021 and related information is as follows: ​
​ | ​ | ​ | ​ | ​ | ​
​ | ​ | ​ | ​ | 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
​ The weighted-average grant date fair value per share of RSUs granted for the three months ended March 31, 2021 and 2020 was $24.43 and $21.34, respectively. The total fair value of RSUs vested (measured on the date of vesting) for the three months ended March 31, 2021 and 2020 was $9,116 and $4,204, respectively. Stock Options A summary of the Company’s stock option activity for the three months ended March 31, 2021 and related information is as follows: ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ |  | ​ |  | ​ | ​ |  | 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
​ The weighted average assumptions used in the Black Scholes option pricing model to determine the fair value of the employee stock option grants were as follows: ​
​ | ​ | ​ | ​ | ​ | ​
​ | 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 | — | % | ​ | — | %
​ The weighted-average grant date fair value per share of stock options granted for the three months ended March 31, 2021 and 2020 was $12.08 and $12.86, respectively. ​ Employee Stock Purchase Plan ​ The Company’s 2015 Employee Stock Purchase Plan allows employees to purchase shares of the Company’s common stock. The purchase price is equal to 85% of the lower of the closing price of the Company’s common stock on (1) the first day of the purchase period or (2) the last day of the purchase period. During the three months ended March 31, 2021, 24,630 shares of common stock were purchased for total proceeds of $358. The expense for the three months ended March 31, 2021 and 2020 was $72 and $79, respectively. ​ Stock-based Compensation Expense ​ A summary of the Company’s stock-based compensation expense was allocated as follows: ​
​ | ​ | ​ | ​ | ​ | ​
​ | 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
​ At March 31, 2021, there was approximately $63,922 of unrecognized compensation expense related to unvested options, restricted stock units and performance stock units, which is expected to be recognized as expense over a weighted average period of approximately 2.9 years.

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