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Income Taxes (Tables)

12 Months Ended

Dec. 31, 2019

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,
		2019				2018				2017
Tax at statutory federal rate		$	(42,997	)		$	(37,557	)		$	(67,145	)
State tax, net of federal benefit			(9,823	)			(6,527	)			(6,203	)
Research and development credits			(4,855	)			(4,775	)			(5,962	)
Stock-based compensation expense			2,906				(2,059	)			3,151
Non-deductible compensation			4,720				901				—
Change in valuation allowance			49,479				50,834				3,241
Impact of the 2017 Tax Act			—				—				74,361
Other			570				(817	)			(1,443	)
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,
		2019				2018
Deferred tax assets:
Net operating loss carryforward		$	210,301			$	172,113
Research and development credits			42,490				35,670
Stock-based compensation			15,628				11,905
Lease liabilities			3,487				—
Other			3,818				3,171
Total gross deferred tax assets			275,724				222,859
Deferred tax liabilities:
Right-of-use lease assets			(3,385	)			—
Total gross deferred tax liabilities			(3,385	)			—
Valuation allowance			(272,339	)			(222,859	)
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,
		2019				2018				2017
Balance at beginning of year		$	2,385			$	120			$	120
Additions for tax positions of prior years			147				—				—
Additions based on tax positions related to current year			2,252				2,265				—
Balance at end of year		$	4,784			$	2,385			$	120

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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.

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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.

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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.

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