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Stock-Based Compensation (Tables) | 12 Months Ended
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Dec. 31, 2018
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract] |
Stock-Based Compensation Expense | The following table shows total stock-based compensation expense recognized in the accompanying consolidated statements of operations (in thousands):
|  | Year Ended December 31, |
|  | 2018 |  |  | 2017 |  |  | 2016 |
Research and development |  | $ | 8,249 |  |  | $ | 5,031 |  |  | $ | 6,463 |
General and administrative |  |  | 6,428 |  |  |  | 4,058 |  |  |  | 8,594 |
Total stock-based compensation expense |  | $ | 14,677 |  |  | $ | 9,089 |  |  | $ | 15,057 |
Assumptions Used for Estimating Fair Value of Employee Stock Options | The assumptions used for estimating the fair value of the employee stock options are as follows:
|  | Year Ended December 31, |
|  | 2018 |  |  | 2017 |  |  | 2016 |
Risk-free interest rate |  | 2.53-2.96% |  |  | 1.81-2.28% |  |  | 1.13-1.61% |
Expected life of option (in years) |  | 5.59-5.61 |  |  | 5.73-5.83 |  |  | 5.28-5.29 |
Expected dividend yield of stock |  |  | 0 | % |  |  | 0 | % |  |  | 0 | %
Expected volatility |  | 0.72-0.75 |  |  | 0.71-0.72 |  |  | 0.68-0.70 |
Weighted-Average Assumptions Used for Estimating Fair Value of ESPP Purchase Rights | The weighted–average assumptions used for estimating the fair value of the ESPP purchase rights are as follows:
|  | Year Ended December 31, |
|  | 2018 |  |  | 2017 |  |  | 2016 |
Risk-free interest rate |  | 2.16-2.80% |  |  | 0.44-0.76% |  |  | 0.41-0.80% |
Expected life of option (in years) |  | 0.5-2 |  |  | 0.5-2.0 |  |  | 0.5-2.0 |
Expected dividend yield of stock |  |  | 0 | % |  |  | 0 | % |  |  | 0 | %
Expected volatility |  | 0.73-0.83 |  |  | 0.66-0.82 |  |  | 0.71-0.76 |

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- Definition Tabular disclosure of the significant assumptions used during the year to estimate the fair value of employee stock purchase plans, including, but not limited to: (a) expected term, (b) expected volatility of the entity's shares, (c) expected dividends, (d) risk-free rate(s), and (e) discount for post-vesting restrictions. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (f)(2) -URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901 + Details
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