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Stock award plan - Assumptions Used (Details) - Stock option	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Valuation assumptions
Expected term (in years)	5 years 9 months
Minimum
Valuation assumptions
Risk-free interest rate (as a percent)	0.34%	1.58%	2.25%
Expected volatility (as a percent)	87.00%	62.00%	64.00%
Expected term (in years)		5 years 9 months	5 years 10 days
Maximum
Valuation assumptions
Risk-free interest rate (as a percent)	1.45%	2.63%	3.10%
Expected volatility (as a percent)	89.00%	92.00%	90.00%
Expected term (in years)		6 years 1 month 9 days	10 years

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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

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (f)(2)(ii)
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The risk-free interest rate assumption that is used in valuing an option on its own shares.

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-Publisher FASB
-Name Accounting Standards Codification
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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.

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