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Employee Benefit Plans (Tables) | 12 Months Ended
--- | ---
Dec. 31, 2020
Employee Benefit Plans |
Assumptions made for the purpose of estimating fair value | ​
​ | ​ | ​ | ​ | ​ | ​ | ​
​ |  | 2020 |  | 2019 |  | 2018
Expected term of options (years) | ​ | 6 | ​ | 6 | ​ | 6
Risk free interest rate | ​ | 0.37% - 1.37% | ​ | 1.52% - 2.53% | ​ | 2.81% - 2.88%
Volatility | ​ | 64.19% - 68.18% | ​ | 69.32% - 87.94% | ​ | 98.31% - 98.89%
Schedule of Share-based Compensation, Stock Options, Activity | ​ A summary of stock option activity for the year December 31, 2020 is as follows (in thousands except share amounts): ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ |  | ​ |  | ​ |  | Weighted |  | ​ | ​
​ | ​ | ​ | ​ | Weighted | ​ | Average | ​ | ​ | ​
​ | ​ | ​ | ​ | Average | ​ | Remaining | ​ | Aggregate
​ | ​ | ​ | ​ | Exercise | ​ | Contractual | ​ | Intrinsic
​ | ​ | Shares | ​ | Price | ​ | Terms | ​ | Value
Options outstanding at December 31, 2019 | ​ | 23,013,590 | ​ | $ | 2.48 | ​ | 6.6 | ​ | $ | 22,277
Granted | ​ | 3,509,549 | ​ | ​ | 12.79 | ​ | — | ​ | ​ | —
Exercised | ​ | (16,159,742) | ​ | ​ | 2.55 | ​ | — | ​ | ​ | —
Forfeited | ​ | (73,249) | ​ | ​ | 6.32 | ​ | — | ​ | ​ | —
Expired | ​ | (5,650) | ​ | ​ | 4.78 | ​ | — | ​ | ​ | —
Options outstanding at December 31, 2020 | ​ | 10,284,498 | ​ | $ | 5.78 | ​ | 7.8 | ​ | $ | 289,316
Options exercisable at December 31, 2020 | ​ | 4,084,124 | ​ | ​ | 2.31 | ​ | 5.8 | ​ | ​ | 129,068
Options unvested at December 31, 2020 | ​ | 6,200,374 | ​ | $ | 8.07 | ​ | 9.1 | ​ | $ | 160,248
Nonvested Restricted Stock Shares Activity | A summary of restricted stock activity for the year ended December 31, 2020 is as follows (in thousands except share amounts): ​
​ | ​ | ​ | ​ | ​ | ​ | ​
​ |  | ​ |  | Aggregate |
​ | ​ | ​ | ​ | Intrinsic | ​
​ | ​ | Shares | ​ | Value | ​
Unvested restricted stock at December 31, 2019 | ​ | 4,608,560 | ​ | $ | — | ​
Granted | ​ | 3,227,149 | ​ | ​ | — | ​
Vested | ​ | (1,896,901) | ​ | ​ | — | ​
Forfeited | ​ | (64,166) | ​ | ​ | ​ | ​
Unvested restricted stock at December 31, 2020 | ​ | 5,874,642 | ​ | $ | 199,209 | ​

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- Definition Tabular disclosure of assumption used to determine benefit obligation and net periodic benefit cost of defined benefit plan. Includes, but is not limited to, discount rate, rate of compensation increase, expected long-term rate of return on plan assets and interest crediting rate. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (k) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920 + Details
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