Income Taxes - U S federal statutory income tax rate (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018
Income Taxes
Income tax benefits - operating losses incurred	$ 0.0	$ 0.0
Income tax benefits - research and development tax credits	$ 0.0	$ 0.0
Federal statutory income tax rate	(21.00%)	(21.00%)
State taxes, net of federal benefit	(6.20%)	(4.80%)
Federal and state research and development tax credits	(6.00%)	(2.90%)
Stock-based compensation expense	0.70%	4.50%
Other	0.50%	0.50%
Increase in deferred tax asset valuation allowance	32.00%	23.70%

The amount of income tax benefits for the net operating losses incurred.

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The amount of income tax benefits for research and development tax credits.

+ References

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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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Percentage of difference between reported income tax expense (benefit) and expected income tax expense (benefit) computed by applying domestic federal statutory income tax rate to pretax income (loss) from continuing operation, attributable to nondeductible expense for share-based payment arrangement.

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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 applicable to state and local income tax expense (benefit), net of federal tax expense (benefit).

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