Note 6 - Stock Option Activity for Employees and Non-employees (Details) - USD ($) $ / shares in Units, $ in Thousands		6 Months Ended	12 Months Ended
		Jun. 30, 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)		114,162
Options Granted, Weighted Average Exercise Price (in dollars per share)		$ 6.77
Options Exercised (in shares)		(72,508)
Options Exercised, Weighted Average Exercise Price (in dollars per share)		$ 1.91
Options Cancelled or forfeited (in shares)		(7,089)
Options Cancelled or forfeited, Weighted Average Exercise Price (in dollars per share)		$ 4.92
Options Outstanding, Ending Balance (in shares)		2,234,934	2,200,369
Options Outstanding, Weighted Average Exercise Price, Ending Balance (in dollars per share)		$ 8.66	$ 8.52
Options Outstanding, Aggregate Intrinsic Value, Ending Balance		$ 4,470	$ 15,705
Options Exercisable (in shares)		997,718
Options Exercisable, Weighted Average Exercise Price (in dollars per share)		$ 5.57
Options Exercisable, Weighted-average Remaining Contractual Term		5 years 328 days
Options Exercisable, Aggregate Intrinsic Value		$ 3,577
Options Vested or Expected to Vest (in shares)	[1]	2,064,955
Options Vested or Expected to Vest, Weighted Average Exercise Price (in dollars per share)	[1]	$ 8.41
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]	$ 4,373

[1] Represents the number of vested options at June 30, 2016 plus the number of unvested options expected to vest based on the unvested options outstanding at June 30, 2016 .