Income Taxes - Effective Tax Rate (Details)	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Tax at federal statutory rate	21.00%	21.00%	34.00%
State tax, net of federal benefit	(0.27%)	1.88%	1.03%
Stock-based compensation	51.45%	(5.84%)	(38.63%)
Warrant expense	0.00%	(6.98%)	(1.00%)
Other items	(4.34%)	(1.46%)	(1.10%)
Research and development credits	13.74%	10.54%	102.35%
Net operating loss expiration	(18.33%)	0.00%	(9.29%)
Change in valuation allowance	(63.42%)	(19.21%)	458.55%
Rate differential impact of Tax Cuts and Jobs Act of 2017	0.00%	0.00%	(545.38%)
Effective tax rate	(0.17%)	(0.07%)	0.53%

Effective Income Tax Rate Reconciliation, Expiration Of Operating Income (Loss)

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Effective Income Tax Rate Reconciliation, Nondeductible Expense, Warrants, Percent

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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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