Note 6 - Stock Option Activity for Employees and Non-employees (Details) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended	12 Months Ended
		Mar. 31, 2016	Dec. 31, 2015
Options Outstanding, Beginning Balance (in shares)		2,200,369
Options Outstanding, Weighted Average Exercise Price, Beginning Balance (in dollars per share)		$ 8.52
Options Outstanding, Weighted-average Remaining Contractual Term, Beginning Balance		7 years 219 days	7 years 292 days
Options Outstanding, Aggregate Intrinsic Value, Beginning Balance		$ 15,705
Options Granted (in shares)		10,000
Options Granted, Weighted Average Exercise Price (in dollars per share)		$ 6.68
Options Exercised (in shares)		(6,394)
Options Exercised, Weighted Average Exercise Price (in dollars per share)		$ 1.92
Options Outstanding, Ending Balance (in shares)		2,203,975	2,200,369
Options Outstanding, Weighted Average Exercise Price, Ending Balance (in dollars per share)		$ 8.53	$ 8.52
Options Outstanding, Aggregate Intrinsic Value, Ending Balance		$ 4,445	$ 15,705
Options Exercisable (in shares)		939,661
Options Exercisable, Weighted Average Exercise Price (in dollars per share)		$ 4.35
Options Exercisable, Weighted-average Remaining Contractual Term		5 years 219 days
Options Exercisable, Aggregate Intrinsic Value		$ 3,657
Options Vested or Expected to Vest (in shares)	[1]	2,021,309
Options Vested or Expected to Vest, Weighted Average Exercise Price (in dollars per share)	[1]	$ 8.22
Options Vested or Expected to Vest, Weighted-average Remaining Contractual Term	[1]	7 years 146 days
Options Vested or Expected to Vest, Aggregate Intrinsic Value	[1]	$ 4,355

[1] Represents the number of vested options at March 31, 2016 plus the number of unvested options expected to vest based on the unvested options outstanding at March 31, 2016.