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Stock-Based Compensation | 3 Months Ended
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Mar. 31, 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 general and administrative expense as follows (in thousands):
|  | Three Months Ended March 31, |
|  | 2019 |  |  | 2018 |
Research and development |  | $ | 9,578 |  |  | $ | 6,370 |
Selling, general and administrative |  |  | 6,852 |  |  |  | 4,508 |
Total stock-based compensation expense |  | $ | 16,430 |  |  | $ | 10,878 |
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 March 31, |  |
|  | 2019 |  |  |  | 2018 |  |
Stock Options |  |  |  |  |  |  |  |  |  |
Expected term (in years) |  |  | 5.3 |  |  |  |  | 5.4 |  |
Expected volatility |  |  | 67.9 |  | % |  |  | 67.9 |  | %
Risk-free interest rate |  |  | 2.6 |  | % |  |  | 2.7 |  | %
Expected dividend yield |  |  | — |  |  |  |  | — |  |
Weighted average estimated fair value |  | $ | 34.14 |  |  |  | $ | 32.18 |  |
ESPPs |  |  |  |  |  |  |  |  |  |
Expected term (in years) |  | 0.5 - 2.0 |  |  |  | 0.5 - 2.0 |  |
Expected volatility |  | 48.1 - 62.1 |  | % |  | 54.9 - 75.3 |  | %
Risk-free interest rate |  | 1.3 - 2.9 |  | % |  | 0.8 - 1.6 |  | %
Expected dividend yield |  |  | — |  |  |  |  | — |  |
Weighted average estimated fair value |  | $ | 20.57 |  |  |  | $ | 10.93 |  |

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- Definition The entire disclosure for compensation-related costs for equity-based compensation, which may include disclosure of policies, compensation plan details, allocation of equity compensation, incentive distributions, equity-based arrangements to obtain goods and services, deferred compensation arrangements, employee stock ownership plan details and employee stock purchase plan details. + 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 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 50 -URI http://asc.fasb.org/subtopic&trid=2208855 + Details
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- References No definition available. + Details
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