Share-based compensation - Assumptions (Details) - $ / shares	9 Months Ended
Sep. 30, 2021	Sep. 30, 2020
Share-based compensation
Expected volatility	83.10%	82.20%
Risk-free interest rate	1.00%	0.60%
Expected term (in years)	6 years 14 days	6 years 3 days
Fair value of common stock	$ 23.61	$ 1.95

Amount of fair value of common stock.

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- References

No definition available.

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- Definition

The estimated measure of the percentage by which a share price is expected to fluctuate during a period. Volatility also may be defined as a probability-weighted measure of the dispersion of returns about the mean. The volatility of a share price is the standard deviation of the continuously compounded rates of return on the share over a specified period. That is the same as the standard deviation of the differences in the natural logarithms of the stock prices plus dividends, if any, over the period.

+ References

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- Definition

The risk-free interest rate assumption that is used in valuing an option on its own shares.

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- Definition

Expected term of award under share-based payment arrangement, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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-Name Accounting Standards Codification
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-Publisher FASB
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