Stock-based compensation - Unrecognized expense and weighted-average period (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Fair value assumptions
Expected dividend yield	0.00%
Employees and directors stock options
Fair value assumptions
Risk-free interest rate	1.00%	2.20%	2.80%
Expected term	6 years	6 years	6 years
Expected volatility	73.70%	74.70%	74.40%
Unvested stock options
Options disclosures
Unrecognized stock-based compensation expense	$ 20.0
Remaining weighted-average remaining vesting period	2 years 6 months
Share-based Payment Arrangement, Employee
Options disclosures
Weighted average fair value of grants (in dollars per share)	$ 7.64

Amount of cost not yet recognized for nonvested award under share-based payment arrangement.

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- Definition

Weighted-average period over which cost not yet recognized is expected to be recognized for 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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- Definition

The estimated dividend rate (a percentage of the share price) to be paid (expected dividends) to holders of the underlying shares over the option's term.

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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.

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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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The weighted average grant-date fair value of options granted during the reporting period as calculated by applying the disclosed option pricing methodology.

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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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