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Stock-Based Compensation (Tables) | 3 Months Ended
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Mar. 31, 2018
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract] |
Schedule of Allocated Stock-Based Compensation Expense | Stock-based compensation expense was allocated to research and development and general and administrative expense as follows (in thousands):
|  | Three Months Ended March 31, |
|  | 2018 |  |  | 2017 |
Research and development |  | $ | 6,370 |  |  | $ | 4,901 |
General and administrative |  |  | 4,508 |  |  |  | 3,508 |
Total stock-based compensation expense |  | $ | 10,878 |  |  | $ | 8,409 |
Schedule of Assumptions used to Estimate Fair Value of Stock Options Granted and Purchases under 2014 Employee Share Purchase Plan | 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, |  |
|  | 2018 |  |  |  | 2017 |  |
Stock Options |  |  |  |  |  |  |  |  |  |
Expected term (in years) |  |  | 5.4 |  |  |  |  | 5.8 |  |
Expected volatility |  |  | 67.9 |  | % |  |  | 71.7 |  | %
Risk-free interest rate |  |  | 2.7 |  | % |  |  | 2.3 |  | %
Expected dividend yield |  |  | — |  |  |  |  | — |  |
Weighted average estimated fair value |  | $ | 32.18 |  |  |  | $ | 16.16 |  |
ESPPs |  |  |  |  |  |  |  |  |  |
Expected term (in years) |  | 0.5 - 2.0 |  |  |  | 0.5 - 2.0 |  |
Expected volatility |  | 54.9 - 75.3 |  | % |  | 63.7 - 77.2 |  | %
Risk-free interest rate |  | 0.8 - 1.6 |  | % |  | 0.5 - 1.0 |  | %
Expected dividend yield |  |  | — |  |  |  |  | — |  |
Weighted average estimated fair value |  | $ | 10.93 |  |  |  | $ | 8.86 |  |

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- Definition Schedule of share based payment award stock options and employee stock purchase plan valuation assumptions. + References No definition available. + Details
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- Definition Tabular disclosure of the allocation of equity-based compensation costs to a given line item on the balance sheet and income statement for the period. This may include the reporting line for the costs and the amount capitalized and expensed. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (h)(1) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5047-113901 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.F) -URI http://asc.fasb.org/extlink&oid=27013229&loc=d3e301413-122809 + Details
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