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Stock-Based Compensation | 6 Months Ended
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Jun. 30, 2021
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract] |
Stock-Based Compensation |
7. | Stock-Based Compensation
Stock-based compensation expense was recorded directly to research and development and selling, general and administrative expense as follows (in thousands):
|  | Three Months Ended June 30, |  |  | Six Months Ended June 30, |
|  | 2021 |  |  | 2020 |  |  | 2021 |  |  | 2020 |
Research and development |  | $ | 10,681 |  |  | $ | 10,780 |  |  | $ | 22,903 |  |  | $ | 21,417 |
Selling, general and administrative |  |  | 8,320 |  |  |  | 6,864 |  |  |  | 15,482 |  |  |  | 13,143 |
Total stock-based compensation expense |  | $ | 19,001 |  |  | $ | 17,644 |  |  | $ | 38,385 |  |  | $ | 34,560 |
The assumptions used to estimate the fair value of stock options granted and purchases under the Company’s 2014 Employee Share Purchase Plan (“ESPP”) using the Black-Scholes option valuation model were as follows:
|  | Three Months Ended June 30, |  |  |  | Six Months Ended June 30, |  |
|  | 2021 |  |  |  | 2020 |  |  |  | 2021 |  |  |  | 2020 |  |
Stock Options |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Expected term (in years) |  |  | 5.6 |  |  |  |  | 5.5 |  |  |  |  | 5.7 |  |  |  |  | 5.7 |  |
Expected volatility |  |  | 63.4 |  | % |  |  | 69.2 |  | % |  |  | 59.4 |  | % |  |  | 68.4 |  | %
Risk-free interest rate |  |  | 0.9 |  | % |  |  | 0.4 |  | % |  |  | 0.7 |  | % |  |  | 0.8 |  | %
Expected dividend yield |  |  | — |  |  |  |  | — |  |  |  |  | — |  |  |  |  | — |  |
Weighted average estimated fair value |  | $ | 11.91 |  |  |  | $ | 21.72 |  |  |  | $ | 25.38 |  |  |  | $ | 17.75 |  |
ESPPs |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Expected term (in years) |  | 0.5 - 2.0 |  |  |  | 0.5 - 2.0 |  |  |  | 0.5 - 2.0 |  |  |  | 0.5 - 2.0 |  |
Expected volatility |  | 47.5 - 104.2 |  | % |  | 49.5 - 77.1 |  | % |  | 47.5 - 104.2 |  | % |  | 49.5 - 77.1 |  | %
Risk-free interest rate |  | 0.0 - 2.2 |  | % |  | 0.2 - 2.9 |  | % |  | 0.0 - 2.2 |  | % |  | 0.2 - 2.9 |  | %
Expected dividend yield |  |  | — |  |  |  |  | — |  |  |  |  | — |  |  |  |  | — |  |
Weighted average estimated fair value |  | $ | 13.60 |  |  |  | $ | 17.82 |  |  |  | $ | 14.69 |  |  |  | $ | 18.11 |  |

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- Definition The entire disclosure for share-based payment arrangement. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -URI http://asc.fasb.org/topic&trid=2228938 + Details
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