Note 9 - Stock-based Compensation - Stock Option Activity for Employees and Non-employees (Details) - USD ($) $ / shares in Units, $ in Thousands		12 Months Ended
		Dec. 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 (Year)		7 years 36 days	7 years 292 days
Options Outstanding, Aggregate Intrinsic Value, Beginning Balance		$ 124	$ 15,705
Options Granted (in shares)		132,495
Options Granted, Weighted Average Exercise Price (in dollars per share)		$ 7.11
Options Exercised (in shares)		(97,521)
Options Exercised, Weighted Average Exercise Price (in dollars per share)		$ 2.01
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)		2,166,254	2,200,369
Options Outstanding, Weighted Average Exercise Price, Ending Balance (in dollars per share)		$ 8.55	$ 8.52
Options Exercisable (in shares)		1,227,575
Options Exercisable, Weighted Average Exercise Price (in dollars per share)		$ 6.80
Options Exercisable, Weighted-average Remaining Contractual Term (Year)		6 years
Options Exercisable, Aggregate Intrinsic Value		$ 124
Options Vested or Expected to Vest (in shares)	[1]	2,052,214
Options Vested or Expected to Vest, Weighted Average Exercise Price (in dollars per share)	[1]	$ 8.37
Options Vested or Expected to Vest, Weighted-average Remaining Contractual Term (Year)	[1]	7 years
Options Vested or Expected to Vest, Aggregate Intrinsic Value	[1]	$ 124

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