Stock based compensation - Black-Scholes options pricing model - (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Options disclosures
Stock-based compensation	$ 2,165	$ 1,422
Stock options granted (in shares)	1,043,800
Employees and directors stock options
Fair value assumptions
Risk-free interest rate	2.00%	1.50%
Expected term	6 years	6 years
Expected volatility	73.60%	73.00%
Options disclosures
Weighted average fair value of grants (in dollars per share)	$ 7.82
Stock-based compensation	$ 1,300
Non-employees stock options
Fair value assumptions
Risk-free interest rate	2.00%	1.50%
Expected term	8 years 8 months 12 days	10 years
Expected volatility	83.60%	84.10%
Options disclosures
Weighted average fair value of grants (in dollars per share)		$ 7.10
Stock-based compensation	$ 200
Unvested stock options
Options disclosures
Unrecognized stock-based compensation expense	$ 14,800
Remaining weighted-average remaining vesting period	3 years 4 months 24 days