Share-based compensation - Assumptions (Details) - $ / shares	3 Months Ended
Mar. 31, 2021	Mar. 31, 2020
Share-based compensation
Expected volatility	83.50%	80.00%
Risk-free interest rate	1.10%	0.80%
Expected life (in years)	6 years 1 month 6 days	5 years 7 months 6 days
Fair value of common stock	$ 31.83	$ 1.62

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
-Publisher FASB
-Name Accounting Standards Codification
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