Significant accounting policies - Stock-based compensation (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Jan. 01, 2017
New Accounting Pronouncement, Early Adoption
Expected dividend yield	0.00%	0.00%	0.00%
Expected term (in years)	5 years 10 months 24 days	6 years	6 years
Weighted-average fair value of options granted	$ 7.19	$ 4.52	$ 2.41
Probability of initial public offering			40.00%
Accumulated deficit	$ (175,888)	$ (144,352)
Minimum
New Accounting Pronouncement, Early Adoption
Risk-free interest rate	2.60%	1.80%	1.20%
Expected volatility	32.40%	46.00%	44.90%
Maximum
New Accounting Pronouncement, Early Adoption
Risk-free interest rate	3.00%	2.10%	1.30%
Expected volatility	36.80%	52.00%	49.00%
ASU No.2016-09 \| Restatement Adjustment
New Accounting Pronouncement, Early Adoption
Accumulated deficit				$ 100

Probability of an initial public offering.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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The cumulative amount of the reporting entity's undistributed earnings or deficit.

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
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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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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 share-based compensation awards, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
-Topic 718
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