Stock-based compensation, Stock Options Plan by Exercise Price Range (Details) - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Outstanding number of shares (in shares)	865,722
Weighted average remaining contractual life	8 years 3 months 22 days
Weighted average exercise price (in dollars per share)	$ 4.74
Exercisable number of shares (in shares)	542,905
Exercisable weighted average exercise price (in dollars per share)	$ 6.10
Weighted average remaining contractual life of exercisable stock options	7 years 10 months 2 days	7 years 6 months 22 days
$1.29 - $5.00 [Member]
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Lower range limit (in dollars per share)	$ 1.29
Upper range limit (in dollars per share)	$ 5.00
Outstanding number of shares (in shares)	483,585
Weighted average remaining contractual life	8 years 10 months 24 days
Weighted average exercise price (in dollars per share)	$ 2.58
Exercisable number of shares (in shares)	160,867
Exercisable weighted average exercise price (in dollars per share)	$ 2.90
$5.01 - $10.00 [Member]
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Lower range limit (in dollars per share)	5.01
Upper range limit (in dollars per share)	$ 10.00
Outstanding number of shares (in shares)	372,500
Weighted average remaining contractual life	7 years 7 months 13 days
Weighted average exercise price (in dollars per share)	$ 6.19
Exercisable number of shares (in shares)	372,500
Exercisable weighted average exercise price (in dollars per share)	$ 6.19
$10.01 - $181.00 [Member]
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Lower range limit (in dollars per share)	10.01
Upper range limit (in dollars per share)	$ 181.00
Outstanding number of shares (in shares)	9,637
Weighted average remaining contractual life	5 years 5 months 8 days
Weighted average exercise price (in dollars per share)	$ 56.65
Exercisable number of shares (in shares)	9,538
Exercisable weighted average exercise price (in dollars per share)	$ 56.35