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Stock-based awards - Stock option valuation (Details) - $ / shares	3 Months Ended	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2019
Assumptions to determine grant-date fair value of stock options granted
Expected term (in years)	6 years 26 days	6 years 15 days
Volatility	71.20%	69.80%
Risk-free interest rate	2.28%	2.39%
Dividend yield	0.00%	0.00%
Weighted average fair value of common stock	$ 9.7200	$ 8.2600

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- References

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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.

+ References

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (f)(2)(iii)
-URI http://asc.fasb.org/extlink&oid=116856206&loc=d3e5070-113901

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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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-Publisher FASB
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-Topic 718
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-Section 50
-Paragraph 2
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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
-Topic 718
-SubTopic 10
-Section 50
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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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Weighted average grant-date fair value of options vested.

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