Stockholders' equity - Schedule of Stock Option Transactions and Number of Stock Options Outstanding (Detail) - $ / shares	6 Months Ended	12 Months Ended
Jun. 30, 2019	Dec. 31, 2018
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Options Outstanding, Number of Shares, beginning balance	2,897,294
Options forfeited, Number of Shares	(266,197)
Options Outstanding, Number of Shares, ending balance	2,631,097	2,897,294
Exercisable, Number of Shares, ending balance	1,160,663
Options Outstanding, Weighted Average Exercise Price, beginning balance	$ 9.04
Options forfeited, Weighted Average Exercise Price	11.76
Options Outstanding, Weighted Average Exercise Price, ending balance	8.76	$ 9.04
Exercisable, Weighted Average Exercise Price, ending balance	$ 11.58
Options Outstanding, Weighted Average Remaining Contractual Life (Years), ending balance	7 years 7 months 6 days	7 years 11 months 15 days
Exercisable, Weighted Average Remaining Contractual Life (Years), ending balance	6 years 3 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.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(1)(iii)
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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.

+ References

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- Definition

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

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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 grantees could have acquired the underlying shares with respect to stock options that were terminated.

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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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