Note 7 - Stock-based Compensation - Stock Option Activity for Employees and Non-employees (Details) - USD ($) $ in Thousands		9 Months Ended	12 Months Ended
		Sep. 30, 2015	Dec. 31, 2014
Outstanding at December 31, 2014 (in shares)			1,235,526
Outstanding at December 31, 2014 (in dollars per share)			$ 4.19
Outstanding at December 31, 2014		7 years 146 days	7 years 36 days
Outstanding at December 31, 2014			$ 7,709
Granted (in shares)		589,924
Granted (in dollars per share)		$ 12.68
Exercised (in shares)		(25,188)
Exercised (in dollars per share)
Outstanding at September 30, 2015 (in shares)		1,800,262
Outstanding at September 30, 2015 (in dollars per share)		$ 7
Outstanding at September 30, 2015		$ 13,061
Exercisable at September 30, 2015 (in shares)		775,789
Exercisable at September 30, 2015 (in dollars per share)		$ 3.22
Exercisable at September 30, 2015		5 years 73 days
Exercisable at September 30, 2015		$ 8,310
Vested or expected to vest at September 30, 2015 (1) (in shares)	[1]	1,683,543
Vested or expected to vest at September 30, 2015 (1) (in dollars per share)	[1]	$ 6.91
Vested or expected to vest at September 30, 2015 (1)	[1]	7 years 109 days
Vested or expected to vest at September 30, 2015 (1)	[1]	$ 12,403

[1] Represents the number of vested options at September 30, 2015 plus the number of unvested options expected to vest based on the unvested options outstanding at September 30, 2015.