Income taxes - Effective Income Tax Reconciliation (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended	12 Months Ended
Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020	Dec. 31, 2019
Income Tax Disclosure [Abstract]
Income tax at U.S. statutory rate of 21%					$ (52,499)	$ (36,582)
Foreign tax rate differential					(889)	(843)
State income taxes (net of federal benefit)					(8,553)	(4,706)
Tax credits					(1,214)	0
Stock-based compensation					66	694
Non-deductible expenses					221	818
Other, net					(1)	5,545
Change in valuation allowance					63,192	35,074
Total provision for income taxes	$ 133	$ 11	$ 226	$ 14	$ 323	$ 0

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Amount of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations.

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Amount 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 increase (decrease) in the valuation allowance for deferred tax assets.

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The amount of income tax expense or benefit for the period computed by applying the domestic federal statutory tax rates to pretax income from continuing operations.

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Amount 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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