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Stock-Based Compensation | 6 Months Ended
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Jun. 30, 2016
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, |
|  | 2016 |  |  | 2015 |  |  | 2016 |  |  | 2015 |
Research and development |  | $ | 5,059 |  |  | $ | 4,434 |  |  | $ | 9,556 |  |  | $ | 8,656 |
General and administrative |  |  | 3,345 |  |  |  | 2,508 |  |  |  | 6,188 |  |  |  | 4,732 |
Total stock-based compensation expense |  | $ | 8,404 |  |  | $ | 6,942 |  |  | $ | 15,744 |  |  | $ | 13,388 |
The assumptions used to estimate the fair value of stock options granted and ESPPs using the Black-Scholes option valuation model were as follows:
|  | Three Months Ended June 30, |  |  | Six Months Ended June 30, |
|  | 2016 |  |  | 2015 |  |  | 2016 |  |  | 2015 |
Stock Options |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Expected term (in years) |  |  | 5.3 |  |  |  | 5.2 |  |  |  | 5.3 |  |  |  | 5.2 |
Expected volatility |  |  | 72 | % |  |  | 70 | % |  |  | 70 | % |  |  | 70 | %
Risk-free interest rate |  |  | 1.3 |  |  |  | 1.7 |  |  |  | 1.4 |  |  |  | 1.7 |
Expected dividend yield |  |  | — |  |  |  | — |  |  |  | — |  |  |  | — |
Weighted average estimated fair value |  | $ | 11.60 |  |  | $ | 12.27 |  |  | $ | 11.48 |  |  | $ | 16.68 |
ESPPs |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Expected term (in years) |  | 0.5 - 2.0 |  |  | 0.4 - 2.0 |  |  | 0.5 - 2.0 |  |  | 0.4 - 2.0 |
Expected volatility |  | 61.9 - 80.7% |  |  | 58.5 - 69.0% |  |  | 61.9 - 80.7% |  |  | 58.5 - 69.0% |
Risk-free interest rate |  | 0.2 - 0.9% |  |  | 0.1 - 0.6% |  |  | 0.2 - 0.9% |  |  | 0.1 - 0.6% |
Expected dividend yield |  |  | — |  |  |  | — |  |  |  | — |  |  |  | — |
Weighted average estimated fair value |  | $ | 10.06 |  |  | $ | 13.06 |  |  | $ | 10.98 |  |  | $ | 13.17 |

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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 -SubTopic 40 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=65895303&loc=d3e17540-113929 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 50 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=68057994&loc=d3e25284-112666 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SAB Topic 14) -URI http://asc.fasb.org/extlink&oid=27013229&loc=d3e301413-122809 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5047-113901 Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5444-113901 + Details
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