Share-based compensation (Details) - $ / shares	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Number of Shares
Outstanding, beginning balance	915,042	555,392
Granted	40,500	665,000
Exercised		(294,667)
Forfeited	(106,312)	(3,619)
Canceled	(25,382)	(7,064)
Outstanding, ending balance	823,848	915,042	555,392
Exercisable, ending balance	392,350	130,042
Weighted Average Exercise Price
Outstanding, beginning balance	$ 4.07	$ 2.70
Granted	4.10	4.59
Exercised		3.26
Forfeited	4.44	2.63
Canceled	4.63	2.63
Outstanding, ending balance	4.01	4.07	$ 2.70
Exercisable, ending balance	$ 3.73	$ 2.80
Remaining Contractual Term in Years
Outstanding	7 years 6 months	8 years 4 months 2 days	7 years 4 months 20 days
Exercisable, ending balance	7 years 3 months 22 days	5 years 8 months 26 days

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The number of shares into which fully or partially vested stock options outstanding as of the balance sheet date can be currently converted under the option plan.

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The weighted-average price as of the balance sheet date at which grantees can acquire the shares reserved for issuance on vested portions of options outstanding and currently exercisable under the stock option plan.

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The number of shares under options that were cancelled during the reporting period as a result of occurrence of a terminating event specified in contractual agreements pertaining to the stock option plan.

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Gross number of share options (or share units) granted during the period.

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The addition or reduction in the number of reserved shares that could potentially be issued under the option plan attributable to reasons other than grants, exercises, forfeitures, and expirations during the reporting period.

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Number of options outstanding, including both vested and non-vested options.

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A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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Weighted average price at which grantees can acquire the shares reserved for issuance under the stock option plan.

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Weighted average price at which option holders acquired shares when converting their stock options into shares.

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Weighted average price at which grantees could have acquired the underlying shares with respect to stock options that were terminated.

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Weighted average per share amount at which grantees can acquire shares of common stock by exercise of options.

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Weighted average price at which grantees could acquire or could have acquired the underlying shares with respect to any other type of change in shares reserved for issuance.

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Weighted average remaining contractual term for vested portions of options outstanding and currently exercisable or convertible, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Weighted average remaining contractual term for option awards outstanding, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Number of share options (or share units) exercised during the current period.

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