Note 6 - Stock-based Compensation - Stock Option Activity for Employees and Non-employees (Details) - USD ($) $ / shares in Units, $ in Thousands		9 Months Ended	12 Months Ended
		Sep. 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 109 days	7 years 292 days
Options Outstanding, Aggregate Intrinsic Value, Beginning Balance		$ 15,705
Options Granted (in shares)		119,162
Options Granted, Weighted Average Exercise Price (in dollars per share)		$ 6.90
Options Exercised (in shares)		(80,825)
Options Exercised, Weighted Average Exercise Price (in dollars per share)		$ 1.91
Options Cancelled or forfeited (in shares)		(69,089)
Options Cancelled or forfeited, Weighted Average Exercise Price (in dollars per share)		$ 14.12
Options Outstanding, Ending Balance (in shares)		2,169,617	2,200,369
Options Outstanding, Weighted Average Exercise Price, Ending Balance (in dollars per share)		$ 8.50	$ 8.52
Options Outstanding, Aggregate Intrinsic Value, Ending Balance		$ 5,814	$ 15,705
Options Exercisable (in shares)		1,055,385
Options Exercisable, Weighted Average Exercise Price (in dollars per share)		$ 5.81
Options Exercisable, Weighted-average Remaining Contractual Term		5 years 292 days
Options Exercisable, Aggregate Intrinsic Value		$ 4,619
Options Vested or Expected to Vest (in shares)	[1]	2,033,481
Options Vested or Expected to Vest, Weighted Average Exercise Price (in dollars per share)	[1]	$ 8.29
Options Vested or Expected to Vest, Weighted-average Remaining Contractual Term	[1]	7 years 73 days
Options Vested or Expected to Vest, Aggregate Intrinsic Value	[1]	$ 5,697

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