Income Taxes - Reconciliation of effective tax rate (Details)	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018
Reconciliation of the U.S. statutory income tax rate to the effective income tax rate
Federal income tax (benefit) at statutory rate	21.00%	21.00%
Permanent differences	(0.57%)	(0.68%)
Federal research and development credits and adjustments	2.42%	4.25%
State income tax benefit, net of federal benefit	5.50%	6.84%
Other	0.49%	(1.41%)
Change in valuation allowance	(28.85%)	(30.00%)
Effective income tax rate	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 deductions. Including, but not limited to, extraterritorial income exclusion deduction, qualified production activity deduction, dividend deduction, deduction for dividend paid to employee stock ownership plan, Medicare prescription drug benefit subsidy deduction, and other deductions.

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