Stock based compensation - Stock Options - (Details) - USD ($)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Options disclosures
Stock-based compensation	$ 4,027,000	$ 425,000
Stock options, all inclusive
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Granted (in shares)	1,030,068	0
Exercised (in shares)	(1,344)
Cancelled or forfeited (in shares)	(6,107)
Outstanding, ending balance (in shares)	1,022,617
Exerciseable (in shares)	94,762
Vested and expected to vest (in shares)	1,022,617
Weighted Average Exercise Price
Granted (in dollars per share)	$ 8.34
Exercised (in dollars per share)	8.39
Cancelled or forfeited (in dollars per share)	7.27
Outstanding (in dollars per share)	8.35
Exerciseable (in dollars per share)	7.51
Vested and expected to vest (in dollars per share)	$ 8.35
Remaining Contractual Life
Outstanding	9 years 6 months
Exerciseable	9 years 2 months 12 days
Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested and Expected to Vest, Outstanding, Weighted Average Remaining Contractual Term	9 years 6 months
Aggregate Instrinsic Value
Oustanding	$ 13,887,000
Exerciseable	1,363,000
Vested and expected to vest	$ 13,887,000
Employees and directors stock options
Options disclosures
Weighted average fair value of grants (in dollars per share)	$ 5.68
Stock-based compensation	$ 720,000
Non-employees stock options
Options disclosures
Weighted average fair value of grants (in dollars per share)	$ 7.95
Stock-based compensation	$ 301,000