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Income Taxes (Tables) | 12 Months Ended
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Dec. 31, 2020
Income Tax Disclosure [Abstract] |
Schedule of Components of Income Tax Expense (Benefit) | The difference between the provision for income taxes and income taxes computed using the effective U.S. federal statutory rate is as follows (in thousands):
|  | December 31, |
|  | 2020 |  |  | 2019 |  |  | 2018 |
Tax at statutory federal rate |  | $ | (47,728 | ) |  | $ | (42,997 | ) |  | $ | (37,557 | )
State tax, net of federal benefit |  |  | (8,218 | ) |  |  | (9,823 | ) |  |  | (6,527 | )
Research and development credits |  |  | (4,327 | ) |  |  | (4,855 | ) |  |  | (4,775 | )
Stock-based compensation expense |  |  | 4,675 |  |  |  | 2,906 |  |  |  | (2,059 | )
Non-deductible compensation |  |  | 1,455 |  |  |  | 4,720 |  |  |  | 901 |
Change in valuation allowance |  |  | 53,621 |  |  |  | 49,479 |  |  |  | 50,834 |
Other |  |  | 522 |  |  |  | 570 |  |  |  | (817 | )
Provision for income taxes |  | $ | — |  |  | $ | — |  |  | $ | — |
Schedule of Deferred Tax Assets and Liabilities | Deferred income tax assets and liabilities arising from differences between accounting for financial statement purposes and tax purposes, less valuation allowance at year-end are as follows (in thousands):
|  | December 31, |
|  | 2020 |  |  | 2019 |
Deferred tax assets: |  |  |  |  |  |  |  |
Net operating loss carryforward |  | $ | 251,971 |  |  | $ | 210,301 |
Research and development credits |  |  | 49,683 |  |  |  | 42,490 |
Stock-based compensation |  |  | 20,211 |  |  |  | 15,628 |
Lease liabilities |  |  | 4,330 |  |  |  | 3,487 |
Other |  |  | 3,777 |  |  |  | 3,818 |
Total gross deferred tax assets |  |  | 329,972 |  |  |  | 275,724 |
Deferred tax liabilities: |  |  |  |  |  |  |  |
Right-of-use lease assets |  |  | (4,014 | ) |  |  | (3,385 | )
Total gross deferred tax liabilities |  |  | (4,014 | ) |  |  | (3,385 | )
Valuation allowance |  |  | (325,958 | ) |  |  | (272,339 | )
Net deferred tax assets |  | $ | — |  |  | $ | — |
Summary of Unrecognized Tax Benefits | A reconciliation of our unrecognized tax benefits is as follows (in thousands):
|  | Year Ended December 31, |
|  | 2020 |  |  | 2019 |  |  | 2018 |
Balance at beginning of year |  | $ | 4,784 |  |  | $ | 2,385 |  |  | $ | 120 |
Additions for tax positions of prior years |  |  | 341 |  |  |  | 147 |  |  |  | — |
Additions based on tax positions related to current year |  |  | 2,281 |  |  |  | 2,252 |  |  |  | 2,265 |
Balance at end of year |  | $ | 7,406 |  |  | $ | 4,784 |  |  | $ | 2,385 |

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- Definition Tabular disclosure of the components of income tax expense attributable to continuing operations for each year presented including, but not limited to: current tax expense (benefit), deferred tax expense (benefit), investment tax credits, government grants, the benefits of operating loss carryforwards, tax expense that results from allocating certain tax benefits either directly to contributed capital or to reduce goodwill or other noncurrent intangible assets of an acquired entity, adjustments of a deferred tax liability or asset for enacted changes in tax laws or rates or a change in the tax status of the entity, and adjustments of the beginning-of-the-year balances of a valuation allowance because of a change in circumstances that causes a change in judgment about the realizability of the related deferred tax asset in future years. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 9 -URI http://asc.fasb.org/extlink&oid=84230637&loc=d3e32639-109319 + Details
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- Definition Tabular disclosure of the components of net deferred tax asset or liability recognized in an entity's statement of financial position, including the following: the total of all deferred tax liabilities, the total of all deferred tax assets, the total valuation allowance recognized for deferred tax assets. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=84230637&loc=d3e32537-109319 + Details
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- Definition Tabular disclosure for tax positions taken in the tax returns filed or to be filed for which it is more likely than not that the tax position will not be sustained upon examination by taxing authorities and other income tax contingencies. Includes, but is not limited to, interest and penalties, reconciliation of unrecognized tax benefits, unrecognized tax benefits that would affect the effective tax rate, tax years that remain subject to examination by tax jurisdictions, and information about positions for which it is reasonably possible that amounts unrecognized will significantly change within 12 months. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 15A -URI http://asc.fasb.org/extlink&oid=84230637&loc=SL6600010-109319 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 15 -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=84230637&loc=d3e32718-109319 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 19 -URI http://asc.fasb.org/extlink&oid=84230637&loc=d3e32840-109319 + Details
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