Stock based compensation - Black-Scholes options pricing model - (Details) - USD ($)	3 Months Ended			6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Mar. 31, 2015	Jun. 30, 2016	Jun. 30, 2015
Options disclosures
Stock-based compensation	$ 1,318,000	$ 902,000		$ 2,739,000	$ 1,204,000
Stock options granted (in shares)				806,010
Employees and directors stock options
Fair value assumptions
Risk-free interest rate	1.30%			1.50%
Expected term	6 years			6 years
Expected volatility	73.50%			73.10%
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
Fair value assumptions
Risk-free interest rate				1.50%
Expected term				10 years
Expected volatility				84.10%
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
Stock options granted (in shares)	0		0
Unvested stock options
Options disclosures
Unrecognized stock-based compensation expense	$ 9,520,000			$ 9,520,000
Remaining weighted-average remaining vesting period				3 years 1 month 6 days