Stock-based compensation - Summary of stock option activity (Details) - USD ($)		9 Months Ended	12 Months Ended
Jun. 14, 2018	Sep. 30, 2018	Dec. 31, 2017
Shares
Shares outstanding, Beginning Balance (shares)		2,580,491
Shares granted (shares)	654,544	1,517,544
Shares exercised (shares)		(97,310)
Shares forfeited (shares)		(714,699)
Shares expired (shares)		(705,387)
Shares outstanding, Ending Balance (shares)		2,580,639	2,580,491
Shares exercisable (shares)		1,126,248
Shares vested or expected to vest (shares)		2,580,639
Weighted-Average Exercise Price
Shares outstanding, weighted-average exercise price, Beginning Balance (in usd per share)		$ 6.65
Shares granted, weighted-average exercise price (in usd per share)		2.74
Shares exercised, weighted-average exercise price (in usd per share)		1.21
Shares forfeited, weighted-average exercise price (in usd per share)		5.96
Shares expired, weighted-average exercise price (in usd per share)		8.35
Shares outstanding, weighted-average exercise price, Ending Balance (in usd per share)		4.28	$ 6.65
Shares exercisable, weighted-average exercise price (in usd per share)		5.92
Shares vested or expected to vest, weighted-average exercise price (in usd per share)		$ 4.28
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]
Outstanding, weighted-average remaining contractual term (in years)		8 years 26 days	7 years 6 months 18 days
Exercisable, weighted-average remaining contractual term (in years)		6 years 6 months 18 days
Vested or expected to vest, weighted-average remaining contractual term (in years)		8 years 26 days
Outstanding, aggregate intrinsic value		$ 0	$ 803,600
Exercisable, aggregate intrinsic value		0
Vested or expected to vest, aggregate intrinsic value		$ 0

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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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Number of options or other stock instruments for which the right to exercise has lapsed under the terms of the plan agreements.

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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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Amount by which the current fair value of the underlying stock exceeds the exercise price of options outstanding.

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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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Amount by which current fair value of underlying stock exceeds exercise price of fully vested and expected to vest options outstanding. Includes, but is not limited to, unvested options for which requisite service period has not been rendered but that are expected to vest based on achievement of performance condition, if forfeitures are recognized when they occur.

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Number of fully vested and expected to vest options outstanding that can be converted into shares under option plan. Includes, but is not limited to, unvested options for which requisite service period has not been rendered but that are expected to vest based on achievement of performance condition, if forfeitures are recognized when they occur.

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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 of the plan that expired.

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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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Amount of difference between fair value of the underlying shares reserved for issuance and exercise price of vested portions of options outstanding and currently exercisable.

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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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Weighted average remaining contractual term for fully vested and expected to vest options outstanding, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days. Includes, but is not limited to, unvested options for which requisite service period has not been rendered but that are expected to vest based on achievement of performance condition, if forfeitures are recognized when they occur.

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

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