Stock-based compensation - Summary of stock option activity (Details)	6 Months Ended
	Jun. 30, 2016 USD ($) $ / shares shares
Shares
Shares outstanding, Beginning Balance | shares	1,824,973
Shares granted | shares	699,820
Shares exercised | shares	(6,401)
Shares canceled or forfeited | shares	(93,375)
Shares outstanding, Ending Balance | shares	2,425,017
Shares exercisable | shares	605,959
Shares vested or expected to vest | shares	2,115,540
Weighted-Average Exercise Price
Shares outstanding, weighted-average exercise price, Beginning Balance (in usd per share) | $ / shares	$ 8.34
Shares granted, weighted-average exercise price (in usd per share) | $ / shares	9.94
Shares exercised, weighted-average exercise price (in usd per share) | $ / shares	1.26
Shares canceled or forfeited, weighted-average exercise price (in usd per share) | $ / shares	12.94
Shares outstanding, weighted-average exercise price, Ending Balance (in usd per share) | $ / shares	8.64
Shares exercisable, weighted-average exercise price (in usd per share) | $ / shares	7.09
Shares vested or expected to vest, weighted-average exercise price (in usd per share) | $ / shares	$ 8.55
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]
Outstanding, weighted-average remaining contractual term (in years)	8 years 10 months 13 days
Exercisable, weighted-average remaining contractual term (in years)	8 years 2 months 27 days
Vested or expected to vest, weighted-average remaining contractual term (in years)	8 years 4 months 13 days
Outstanding, aggregate intrinsic value | $	$ 6,528,784
Exercisable, aggregate intrinsic value | $	2,864,265
Vested or expected to vest, aggregate intrinsic value | $	$ 6,034,998