Income Taxes - Tax Rates (Details)	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Taxes
Statutory federal tax rate	(21.00%)	(21.00%)	(34.00%)
State income taxes, net of federal benefit	(2.00%)	(3.00%)	(3.00%)
Research and development and other tax credits	(3.00%)	(3.00%)	(2.00%)
Other	1.00%	1.00%	(1.00%)
Change in tax rate	3.00%	5.00%	70.00%
Change in valuation allowance	22.00%	21.00%	(30.00%)
Income tax provision	0.00%	0.00%	0.00%

Percentage of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations.

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Percentage of domestic federal statutory tax rate applicable to pretax income (loss).

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Percentage of the difference between reported income tax expense (benefit) and expected income tax expense (benefit) computed by applying the domestic federal statutory income tax rates to pretax income (loss) from continuing operations attributable to changes in the valuation allowance for deferred tax assets.

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