Stock based compensation - Black-Scholes options pricing model - (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Options disclosures
Stock-based compensation	$ 1,789,000	$ 857,000	$ 4,529,000	$ 2,061,000
Stock options granted (in shares)			896,610
Employees and directors stock options
Fair value assumptions
Risk-free interest rate	1.20%	1.70%	1.50%	1.60%
Expected term	6 years	6 years	6 years	6 years
Expected volatility	73.20%	78.40%	73.10%	78.60%
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
Unvested stock options
Options disclosures
Unrecognized stock-based compensation expense	$ 9,413,000		$ 9,413,000
Remaining weighted-average remaining vesting period			2 years 10 months 24 days