Income taxes - Effective income tax reconciliation (Details)	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Operating Loss Carryforwards [Line Items]
U.S. federal statutory income tax rate	(21.00%)	(21.00%)	(35.00%)
Foreign tax rate differential	(4.00%)	(4.00%)	10.00%
Not taxable government grants	(5.40%)	(7.50%)	(3.00%)
Stock-based compensation	(0.30%)	(4.60%)	(3.50%)
Other	(0.30%)	(0.50%)
Change in deferred tax asset valuation allowance	31.00%	37.50%	31.50%
Effective income tax rate	0.00%	(0.10%)	0.00%
Austria
Operating Loss Carryforwards [Line Items]
U.S. federal statutory income tax rate	(25.00%)

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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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 statutory income tax expense (benefit) outside of the country of domicile.

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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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