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Stock-Based Compensation | 6 Months Ended
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Jun. 30, 2017
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract] |
Stock-Based Compensation |
5. | 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 June 30, |  |  | Six Months Ended June 30, |
|  | 2017 |  |  | 2016 |  |  | 2017 |  |  | 2016 |
Research and development |  | $ | 5,621 |  |  | $ | 5,059 |  |  | $ | 10,521 |  |  | $ | 9,556 |
General and administrative |  |  | 3,950 |  |  |  | 3,345 |  |  |  | 7,459 |  |  |  | 6,188 |
Total stock-based compensation expense |  | $ | 9,571 |  |  | $ | 8,404 |  |  | $ | 17,980 |  |  | $ | 15,744 |
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, |  |
|  | 2017 |  |  |  | 2016 |  |  |  | 2017 |  |  |  | 2016 |  |
Stock Options |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Expected term (in years) |  |  | 5.4 |  |  |  |  | 5.3 |  |  |  |  | 5.7 |  |  |  |  | 5.3 |  |
Expected volatility |  |  | 71.0 |  | % |  |  | 71.7 |  | % |  |  | 71.6 |  | % |  |  | 69.8 |  | %
Risk-free interest rate |  |  | 1.9 |  |  |  |  | 1.3 |  |  |  |  | 2.3 |  |  |  |  | 1.4 |  |
Expected dividend yield |  |  | — |  |  |  |  | — |  |  |  |  | — |  |  |  |  | — |  |
Weighted average estimated fair value |  | $ | 17.06 |  |  |  | $ | 11.60 |  |  |  | $ | 16.26 |  |  |  | $ | 11.48 |  |
ESPPs |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Expected term (in years) |  | 0.5 - 2.0 |  |  |  | 0.5 - 2.0 |  |  |  | 0.5 - 2.0 |  |  |  | 0.5 - 2.0 |  |
Expected volatility |  | 52.8 - 77.2 |  | % |  | 69.1 - 80.7 |  | % |  | 52.8 - 77.2 |  | % |  | 61.9 - 80.7 |  | %
Risk-free interest rate |  | 0.5 - 1.3 |  | % |  | 0.2 - 0.9 |  | % |  | 0.5 - 1.3 |  | % |  | 0.2 - 0.9 |  | %
Expected dividend yield |  |  | — |  |  |  |  | — |  |  |  |  | — |  |  |  |  | — |  |
Weighted average estimated fair value |  | $ | 9.10 |  |  |  | $ | 10.60 |  |  |  | $ | 8.98 |  |  |  | $ | 10.98 |  |

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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://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -URI http://asc.fasb.org/topic&trid=2228938 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 50 -URI http://asc.fasb.org/subtopic&trid=2208855 + Details
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