Note 6 - Stock-based Compensation - Stock Option Activity for Employees and Non-employees (Details) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended	12 Months Ended
		Mar. 31, 2017	Dec. 31, 2016
Options Outstanding, Beginning Balance (in shares)		2,166,254
Options Outstanding, Weighted Average Exercise Price, Beginning Balance (in dollars per share)		$ 8.55
Options Outstanding, Weighted-average Remaining Contractual Term, Beginning Balance (Year)		7 years 219 days	7 years 36 days
Options Outstanding, Aggregate Intrinsic Value, Beginning Balance		$ 64	$ 124
Options Granted (in shares)		659,342
Options Granted, Weighted Average Exercise Price (in dollars per share)		$ 2.05
Options Exercised (in shares)		(74,001)
Options Exercised, Weighted Average Exercise Price (in dollars per share)		$ 1.45
Options Outstanding, Ending Balance (in shares)		2,751,595	2,166,254
Options Outstanding, Weighted Average Exercise Price, Ending Balance (in dollars per share)		$ 7.18	$ 8.55
Options Exercisable (in shares)		1,242,299
Options Exercisable, Weighted Average Exercise Price (in dollars per share)		$ 7.35
Options Exercisable, Weighted-average Remaining Contractual Term (Year)		6 years
Options Exercisable, Aggregate Intrinsic Value		$ 64
Options Vested or Expected to Vest (in shares)	[1]	2,751,595
Options Vested or Expected to Vest, Weighted Average Exercise Price (in dollars per share)	[1]	$ 7.18
Options Vested or Expected to Vest, Weighted-average Remaining Contractual Term (Year)	[1]	7 years 219 days
Options Vested or Expected to Vest, Aggregate Intrinsic Value	[1]	$ 64

[1] Represents the number of vested options at March 31, 2017 plus the number of unvested options expected to vest based on the unvested options outstanding at March 31, 2017.