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Equity Compensation Plans (Tables) | 12 Months Ended
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Dec. 31, 2019
Equity Compensation Plans |
Summary of stock option activity | ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ |  | ​ |  | ​ | ​ |  | Weighted |  | ​ | ​
​ | ​ | ​ | ​ | Weighted | ​ | Average |  | ​ | ​
​ | ​ | Number of | ​ | Average | ​ | Remaining |  | Aggregate
​ | ​ | Shares | ​ | Exercise | ​ | Contractual |  | Intrinsic
Time-Based Stock Options | ​ | Outstanding | ​ | Price | ​ | Term(Years) |  | Value
Outstanding at December 31, 2017: |  | 4,304,201 |  | $ | 7.28 |  | 5.39 | ​ | $ | 90,473
Granted |  | 656,250 |  | ​ | 29.48 | ​ | ​ | ​ | ​ | ​
Forfeited |  | (40,473) |  | ​ | 9.56 | ​ | ​ | ​ | ​ | ​
Exercised |  | (1,026,807) |  | ​ | 3.72 | ​ | ​ | ​ | ​ | ​
Outstanding at December 31, 2018: | ​ | 3,893,171 |  | $ | 11.94 |  | 5.95 | ​ | $ | 41,020
Granted | ​ | 344,176 | ​ | ​ | 18.07 | ​ | ​ | ​ | ​ | ​
Forfeited | ​ | (287,609) | ​ | ​ | 22.75 | ​ | ​ | ​ | ​ | ​
Exercised | ​ | (529,557) | ​ | ​ | 5.21 | ​ | ​ | ​ | ​ | ​
Outstanding at December 31, 2019 |  | 3,420,181 | ​ | $ | 12.69 | ​ | 5.70 | ​ | $ | 26,074
Vested and expected to vest | ​ | 3,420,181 | ​ | $ | 12.69 | ​ | 5.68 | ​ | $ | 26,074
Exercisable at December 31, 2019 |  | 2,099,616 | ​ | $ | 7.90 | ​ | 4.10 | ​ | $ | 22,712
Summary of stock unit activity | ​
​ | ​ | ​ | ​ | ​ | ​
​ | ​ | ​ | ​ | Weighted
​ | ​ | Number of | ​ | Average
​ | ​ | Shares | ​ | Grant Date
Restricted and Performance Stock Units | ​ | Outstanding | ​ | Fair Value
Outstanding at December 31, 2017: |  | 610,730 |  | $ | 21.14
Granted |  | 516,433 | ​ | ​ | 23.34
Released | ​ | (7,150) | ​ | ​ | 8.95
Forfeited |  | (13,650) | ​ | ​ | 25.89
Outstanding at December 31, 2018: | ​ | 1,106,363 | ​ | ​ | 22.18
Granted | ​ | 217,146 | ​ | ​ | 17.60
Released | ​ | (86,405) | ​ | ​ | 16.77
Forfeited | ​ | (123,407) | ​ | ​ | 22.97
Outstanding at December 31, 2019 |  | 1,113,697 | ​ | ​ | 21.62
Schedule of weighted-average assumptions for options granted | ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
Year ended December 31, |  | 2019 | ​ | 2018 |  | 2017 |
​ | ​ | ​ | ​
Expected term (in years) |  | 5.76 | ​ | 6.22 | ​ | 6.16 | ​
Expected volatility |  | 54.97 | % | 50.99 | % | 50.43 | %
Risk free rate |  | 1.71 | % | 2.70 | % | 2.12 | %
Expected dividends |  | — | % | — | % | — | %
Summary of weighted average estimated grant date fair value per share | ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | 2019 | ​ | 2018 | ​ | 2017
Stock options | ​ | $ | 18.07 | ​ | $ | 15.05 | ​ | $ | 8.41
Restricted and performance stock units | ​ | ​ | 17.60 | ​ | ​ | 23.34 | ​ | ​ | 26.24

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