Note 9 - Stock Option Activity for Employees and Non-employees (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2015	Dec. 31, 2014
Options Outstanding, Beginning Balance (in shares)		1,235,526
Options Outstanding, Weighted Average Exercise Price, Beginning Balance (in dollars per share)		$ 4.19
Options Outstanding, Weighted-average Remaining Contractual Term, Beginning Balance		7 years 292 days	7 years 36 days
Options Outstanding, Aggregate Intrinsic Value, Beginning Balance		$ 7,709
Options Granted (in shares)		1,010,256
Share-based Compensation Arrangements by Share-based Payment Award, Options, Grants in Period, Weighted Average Exercise Price		$ 13.52
Options Exercised (in shares)		(45,413)
Options Exercised, Weighted Average Exercise Price (in dollars per share)		$ 1.96
Options Outstanding, Ending Balance (in shares)		2,200,369	1,235,526
Options Outstanding, Weighted Average Exercise Price, Ending Balance (in dollars per share)		$ 8.52	$ 4.19
Options Outstanding, Aggregate Intrinsic Value, Ending Balance		$ 15,705	$ 7,709
Options Exercisable (in shares)		812,891
Options Exercisable, Weighted Average Exercise Price (in dollars per share)		$ 3.53
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Weighted Average Remaining Contractual Term		5 years 109 days
Options Exercisable, Aggregate Intrinsic Value		$ 9,755
Options Vested or Expected to Vest (in shares)	[1]	1,991,131
Options Vested or Expected to Vest, Weighted Average Exercise Price (in dollars per share)	[1]	$ 8.16
Options Vested or Expected to Vest, Weighted-average Remaining Contractual Term	[1]	7 years 219 days
Options Vested or Expected to Vest, Aggregate Intrinsic Value	[1]	$ 14,926

[1] Represents the number of vested options at December 31, 2015 plus the number of unvested options expected to vest based on the unvested options outstanding at December 31, 2015.