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Stock-Based Compensation | 6 Months Ended
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Jun. 30, 2019
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract] |
Stock-Based Compensation |
6. | Stock-Based Compensation
Stock-based compensation expense was allocated to research and development and selling, general and administrative expense as follows (in thousands):
|  | Three Months Ended June 30, |  |  | Six Months Ended June 30, |
|  | 2019 |  |  | 2018 |  |  | 2019 |  |  | 2018 |
Research and development |  | $ | 10,450 |  |  | $ | 7,894 |  |  | $ | 20,028 |  |  | $ | 14,264 |
Selling, general and administrative |  |  | 7,192 |  |  |  | 5,337 |  |  |  | 14,044 |  |  |  | 9,845 |
Total stock-based compensation expense |  | $ | 17,642 |  |  | $ | 13,231 |  |  | $ | 34,072 |  |  | $ | 24,109 |
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, |  |
|  | 2019 |  |  |  | 2018 |  |  |  | 2019 |  |  |  | 2018 |  |
Stock Options |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Expected term (in years) |  |  | 5.3 |  |  |  |  | 5.4 |  |  |  |  | 5.3 |  |  |  |  | 5.4 |  |
Expected volatility |  |  | 68.2 |  | % |  |  | 67.5 |  | % |  |  | 67.9 |  | % |  |  | 67.8 |  | %
Risk-free interest rate |  |  | 2.0 |  | % |  |  | 2.8 |  | % |  |  | 2.5 |  | % |  |  | 2.7 |  | %
Expected dividend yield |  |  | — |  |  |  |  | — |  |  |  |  | — |  |  |  |  | — |  |
Weighted average estimated fair value |  | $ | 24.40 |  |  |  | $ | 31.35 |  |  |  | $ | 33.20 |  |  |  | $ | 32.12 |  |
ESPPs |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Expected term (in years) |  | 0.5 - 2.0 |  |  |  | 0.5 - 2.0 |  |  |  | 0.5 - 2.0 |  |  |  | 0.5 - 2.0 |  |
Expected volatility |  | 48.1 - 62.1 |  | % |  | 47.3 - 75.3 |  | % |  | 48.1 - 62.1 |  | % |  | 47.3 - 75.3 |  | %
Risk-free interest rate |  | 1.3 - 2.9 |  | % |  | 0.8 - 2.6 |  | % |  | 1.3 - 2.9 |  | % |  | 0.8 - 2.6 |  | %
Expected dividend yield |  |  | — |  |  |  |  | — |  |  |  |  | — |  |  |  |  | — |  |
Weighted average estimated fair value |  | $ | 19.07 |  |  |  | $ | 15.25 |  |  |  | $ | 19.65 |  |  |  | $ | 13.67 |  |

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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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