Income taxes - Reconciliation of the federal statutory income tax rate to the effective tax rate (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018
Reconciliation of the federal statutory income tax rate to the effective tax rate
Federal statutory income tax rate (in percent)	21.00%	21.00%
State taxes, net of federal benefit (in percent)	3.20%	6.00%
Tax credits (in percent)	2.30%	2.70%
Share-based compensation (in percent)	2.30%	1.10%
Permanent items (in percent)	(0.90%)	(1.20%)
Defereed tax rate changes	(1.40%)
Change in valuation allowance (in percent)	(24.60%)	(29.90%)
Other (in percent)	(1.40%)	0.20%
Effective income tax rate (in percent)	0.50%	(0.10%)
Aushon acquisition
Income Taxes
Amortizable goodwill		$ 0.4

Effective Income Tax Rate Reconciliation Deferred Tax Rate Changes

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The amount of goodwill related to historical tax basis.

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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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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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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 tax credits. Including, but not limited to, research credit, foreign tax credit, investment tax credit, and other tax credits.

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