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Stock-Based Compensation (Tables) | 12 Months Ended
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Dec. 31, 2019
Disclosure of Compensation Related Costs, Share-based 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, |  |  |  |
| 2019 |  | 2018 |  | 2017
Research and development | $ | 10,135 |  |  | $ | 8,249 |  |  | $ | 5,031 |
General and administrative | 9,195 |  |  | 6,428 |  |  | 4,058 |
Total stock-based compensation expense | $ | 19,330 |  |  | $ | 14,677 |  |  | $ | 9,089 |
Assumptions Used for Estimating Fair Value of Employee Stock Options | The assumptions used for estimating the fair value of the employee stock options were as follows:
| Year Ended December 31, |  |  |  |
| 2019 |  | 2018 |  | 2017
Risk-free interest rate | 1.68-2.25% |  |  | 2.53-2.96% |  |  | 1.81-2.28%
Expected term (in years) | 5.50-5.62 |  | 5.59-5.61 |  | 5.73-5.83
Expected dividend yield of stock | — |  |  | — |  |  | — |
Expected volatility | 76.46-78.39% |  | 72.33-75.49% |  | 71.11-72.30%
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, |  |  |  |
| 2019 |  | 2018 |  | 2017
Risk-free interest rate | 1.53-2.42% |  | 2.16-2.80% |  | 0.44-0.76%
Expected term (in years) | 0.5 – 2.0 |  | 0.5-2.0 |  | 0.5-2.0
Expected dividend yield of stock | — |  |  | — |  |  | — |
Expected volatility | 51.02-91.96% |  | 73.21-83.25% |  | 66.39-82.19%

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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://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (h)(1) -URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -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=115993241&loc=d3e301413-122809 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5047-113901 + Details
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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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