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Stock award plan - (Tables) | 12 Months Ended
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Dec. 31, 2020
Share-based Payment Arrangement [Abstract] |
Summary of stock option activity | A summary of stock option activity is as follows: ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ |  | ​ |  | ​ | ​ |  | Weighted- |  |  | ​ | ​
​ | ​ | ​ | ​ | Weighted- | ​ | average | ​ | ​ | Aggregate
​ | ​ | ​ | ​ | average | ​ | remaining | ​ | ​ | intrinsic
​ | ​ | Number of | ​ | exercise | ​ | contractual | ​ | ​ | value(in
​ | ​ | options | ​ | price | ​ | term | ​ | ​ | thousands)
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ |  | ​ | ​ | ​
Outstanding at December 31, 2017 |  | 6,448,642 | ​ | $ | 29.00 |  |  |  | ​ | ​ |
Granted |  | 3,181,623 | ​ | $ | 26.64 |  |  |  | ​ | ​ |
Exercised |  | (633,973) | ​ | $ | 18.61 |  |  |  | ​ | ​ |
Forfeited |  | (461,934) | ​ | $ | 35.36 |  |  |  | ​ | ​ |
Outstanding at December 31, 2018 |  | 8,534,358 | ​ | $ | 28.58 |  |  |  | ​ | ​ |
Granted |  | 3,977,995 | ​ | $ | 35.81 |  |  |  | ​ | ​ |
Exercised |  | (949,887) | ​ | $ | 19.25 |  |  |  | ​ | ​ |
Forfeited |  | (518,527) | ​ | $ | 35.27 |  |  |  | ​ | ​ |
Outstanding at December 31, 2019 |  | 11,043,939 | ​ | $ | 31.67 |  |  |  | ​ | ​ |
Granted |  | 2,777,975 | ​ | $ | 51.06 |  |  |  | ​ | ​ |
Exercised |  | (3,268,452) | ​ | $ | 24.25 |  |  |  | ​ | ​ |
Forfeited/Cancelled |  | (889,785) | ​ | $ | 42.14 |  |  |  | ​ | ​ |
Outstanding at December 31, 2020 |  | 9,663,677 | ​ | $ | 38.72 |  | 7.37 | years | ​ | $ | 218,374
Vested or Expected to vest at December 31, 2020 |  | 4,910,488 | ​ | $ | 41.24 |  | 8.49 | years | ​ | $ | 97,320
Exercisable at December 31, 2020 |  | 4,249,063 | ​ | $ | 35.24 |  | 5.92 | years | ​ | $ | 112,228
​
Schedule of assumptions used to estimate fair values of grants made on the date of grant | The fair values of grants made in the years ended December 31, 2020, 2019 and 2018 were contemporaneously estimated on the date of grant using the following assumptions: ​
​ | ​ | ​ | ​ | ​ | ​ | ​
​ |  | ​ | ​ | ​ | ​ | ​
​ |  | 2020 |  | 2019 |  | 2018
Risk-free interest rate |  | 0.34% - 1.45% |  | 1.58% - 2.63% |  | 2.25% - 3.10%
Expected volatility |  | 87% - 89% |  | 62% - 92% |  | 64% - 90%
Expected term |  | 5.75 years |  | 5.75 - 6.11 years |  | 5.03 - 10.00 years
​
Summary of information on the Company's restricted stock | The following table summarizes information on the Company’s restricted stock awards and units: ​
​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Restricted Stock Awards and Units
​ | ​ | ​ | ​ | Weighted
​ | ​ | ​ | ​ | Average
​ | ​ | ​ | ​ | Grant
​ | ​ | Number of | ​ | Date
​ |  | Shares |  | Fair Value
Unvested at December 31, 2019 | ​ | 642,419 | ​ | $ | 24.50
Granted |  | 663,045 | ​ | $ | 51.32
Vested |  | (232,975) | ​ | $ | 23.87
Forfeited |  | (90,431) | ​ | $ | 36.44
Unvested at December 31, 2020 |  | 982,058 | ​ | $ | 41.78
​
Schedule of share-based compensation expense recorded in the statement of operations | The Company recorded share-based compensation expense in the statement of operations related to incentive stock options, nonstatutory stock options, restricted stock awards, restricted stock units and the ESPP as follows: ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Year ended December 31,
​ | ​ | 2020 |  | 2019 |  | 2018
Research and development | ​ | $ | 38,716 | ​ | $ | 20,836 | ​ | $ | 16,096
Selling, general and administrative | ​ |  | 31,609 | ​ |  | 21,298 | ​ |  | 17,156
Total | ​ | $ | 70,325 | ​ | $ | 42,134 | ​ | $ | 33,252
​

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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=120381028&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=120381028&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=120381028&loc=d3e5070-113901 + Details
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