Stock based compensation - Stock Options - (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Outstanding, beginning balance (in shares)			1,022,617
Granted (in shares)			896,610
Exercised (in shares)			(19,618)
Cancelled or forfeited (in shares)			(36,990)
Outstanding, ending balance (in shares)	1,862,619		1,862,619
Exerciseable (in shares)	388,345		388,345
Vested and expected to vest (in shares)	1,862,619		1,862,619
Weighted Average Exercise Price
Oustanding (in dollars per share)			$ 8.35
Granted (in dollars per share)			11.83
Exercised (in dollars per share)			7.50
Cancelled or forfeited (in dollars per share)			10.07
Oustanding (in dollars per share)	$ 10.01		10.01
Exercisable (in dollars per share)	8.65		8.65
Vested and expected to vest (in dollars per share)	$ 10.01		$ 10.01
Remaining Contractual Life
Outstanding			9 years 1 month 6 days
Exerciseable			8 years 8 months 12 days
Vested and expected to vest			9 years 1 month 6 days
Aggregate Intrinsic Value
Oustanding	$ 4,353,000		$ 4,353,000
Exerciseable	1,311,000		1,311,000
Vested and expected to vest	4,353,000		4,353,000
Options disclosures
Stock-based compensation	$ 1,789,000	$ 857,000	$ 4,529,000	$ 2,061,000
Employees and directors stock options
Options disclosures
Weighted average fair value of grants (in dollars per share)	$ 8.98		$ 7.62
Stock-based compensation	$ 914,000		$ 2,155,000	$ 215,000
Non-employees stock options
Options disclosures
Weighted average fair value of grants (in dollars per share)			$ 9.21	$ 6.23
Stock-based compensation	$ 56,000	$ 49,000	$ 110,000	$ 263,000