Stock based compensation - Stock Options - (Details) - USD ($)	3 Months Ended			6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Mar. 31, 2015	Jun. 30, 2016	Jun. 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)				806,010
Exercised (in shares)				(12,910)
Cancelled or forfeited (in shares)				(31,766)
Outstanding, ending balance (in shares)	1,783,951			1,783,951
Exerciseable (in shares)	266,194			266,194
Vested and expected to vest (in shares)	1,783,951			1,783,951
Weighted Average Exercise Price
Oustanding (in dollars per share)				$ 8.35
Granted (in dollars per share)				11.58
Exercised (in dollars per share)				7.27
Cancelled or forfeited (in dollars per share)				9.93
Oustanding (in dollars per share)	$ 9.80			9.80
Exercisable (in dollars per share)	8.34			8.34
Vested and expected to vest (in dollars per share)	$ 9.80			$ 9.80
Remaining Contractual Life
Outstanding				9 years 3 months 18 days
Exerciseable				8 years 8 months 12 days
Vested and expected to vest				9 years 3 months 18 days
Aggregate Intrinsic Value
Oustanding	$ 2,817,000			$ 2,817,000
Exerciseable	709,000			709,000
Vested and expected to vest	2,817,000			2,817,000
Options disclosures
Stock-based compensation	$ 1,318,000	$ 902,000		$ 2,739,000	$ 1,204,000
Employees and directors stock options
Options disclosures
Weighted average fair value of grants (in dollars per share)	$ 8.27			$ 7.48	$ 4.93
Stock-based compensation	$ 690,000			$ 1,240,000	$ 137,000
Non-employees stock options
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Granted (in shares)	0		0
Options disclosures
Weighted average fair value of grants (in dollars per share)				$ 7.10	$ 6.08
Stock-based compensation	$ 42,000			$ 54,000	$ 215,000