Stock-based compensation - Summary of stock option activity (Details) (USD $)	3 Months Ended
Mar. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Shares outstanding, Beginning Balance	926,832
Shares granted	287,743
Shares exercised	(37,064)
Shares canceled or forfeited	(23,350)
Shares outstanding, Ending Balance	1,154,161
Shares exercisable	13,017
Shares vested or expected to vest	1,084,741
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Abstract]
Shares outstanding, weighted-average exercise price, Ending Balance (in usd per share)	$ 3.40
Shares granted, weighted-average exercise price (in usd per share)	$ 13.19
Shares exercised, weighted-average exercise price (in usd per share)	$ 10.79
Shares canceled or forfeited, weighted-average exercise price (in usd per share)	$ 10.79
Shares outstanding, weighted-average exercise price, Beginning Balance (in usd per share)	$ 5.46
Shares exercisable, weighted-average exercise price (in usd per share)	$ 0.61
Shares vested or expected to vest, weighted-average exercise price (in usd per share)	$ 5.37
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]
Outstanding, weighted-average remaining contractual term	9 years 6 months
Exercisable, weighted-average remaining contractual term	9 years 15 days
Vested or expected to vest, weighted-average remaining contractual term	9 years 6 months
Outstanding, aggregate intrinsic value	$ 16,324,513
Exercisable, aggregate intrinsic value	247,174
Vested or expected to vest, aggregate intrinsic value	$ 15,431,131

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

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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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Amount by which the current fair value of the underlying stock exceeds the exercise price of fully vested and expected to vest options outstanding.

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As of the balance sheet date, the number of shares into which fully vested and expected to vest stock options outstanding can be converted under the option plan.

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As of the balance sheet date, the weighted-average exercise price for outstanding stock options that are fully vested or expected to vest.

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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 the reported fact of one year, five months, and thirteen days.

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

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