Stock-based compensation - Unrecognized expense and weighted-average period (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
Mar. 31, 2021	Mar. 31, 2020
Options disclosures
Stock-based compensation.	$ 3,601	$ 4,040
Stock options
Options disclosures
Stock-based compensation.	2,523	$ 3,208
Unrecognized stock-based compensation expense	$ 22,000
Remaining weighted-average remaining vesting period	2 years 9 months 18 days
Share-based Payment Arrangement, Employee \| Stock options
Fair value assumptions
Risk-free interest rate	0.70%	1.40%
Expected term	6 years	6 years
Expected volatility	74.80%	72.30%
Options disclosures
Weighted average fair value of grants (in dollars per share)	$ 4.81
Stock-based compensation.	$ 2,500	$ 3,200

Amount of expense for award under share-based payment arrangement. Excludes amount capitalized.

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

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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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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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