Note 8 - Stock-based Compensation - Stock Option Activity for Employees and Non-employees (Details) - USD ($) $ in Thousands		6 Months Ended	12 Months Ended
		Jun. 30, 2015	Dec. 31, 2014
Outstanding at December 31, 2014 (in shares)		1,757,959	1,235,526
Outstanding at December 31, 2014 (in dollars per share)		$ 6.62	$ 4.19
Outstanding at December 31, 2014		7 years 182 days	7 years 36 days
Outstanding at December 31, 2014		$ 19,772	$ 7,709
Granted (in shares)		526,591
Granted (in dollars per share)		$ 12.29
Exercised (in shares)		(4,158)
Exercised (in dollars per share)		$ 1.83
Exercisable at June 30, 2015 (in shares)		751,204
Exercisable at June 30, 2015 (in dollars per share)		$ 3.14
Exercisable at June 30, 2015		5 years 73 days
Exercisable at June 30, 2015		$ 11,066
Vested or expected to vest at June 30, 2015 (1) (in shares)	[1]	1,650,391
Vested or expected to vest at June 30, 2015 (1) (in dollars per share)	[1]	$ 6.48
Vested or expected to vest at June 30, 2015 (1)	[1]	7 years 146 days
Vested or expected to vest at June 30, 2015 (1)	[1]	$ 18,802

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