Note 7 - 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, 2019	Dec. 31, 2018
Options Outstanding, Beginning Balance (in shares)		4,597,226
Options Outstanding, Weighted Average Exercise Price, Beginning Balance (in dollars per share)		$ 5.12
Options Outstanding, Weighted-average Remaining Contractual Term (Year)		7 years 146 days	7 years 146 days
Options Outstanding, Aggregate Intrinsic Value			$ 404
Options Granted (in shares)		1,182,500
Options Granted, Weighted Average Exercise Price (in dollars per share)		$ 2.66
Options Exercised (in shares)
Options Exercised, Weighted Average Exercise Price (in dollars per share)
Options Forfeited (in shares)
Options Forfeited, Weighted Average Exercise Price (in dollars per share)
Options Expired (in shares)
Options Expired, Weighted Average Exercise Price (in dollars per share)
Options Outstanding, Ending Balance (in shares)		5,779,726	4,597,226
Options Outstanding, Weighted Average Exercise Price, Ending Balance (in dollars per share)		$ 4.62	$ 5.12
Options Exercisable (in shares)		2,606,279
Options Exercisable, Weighted Average Exercise Price (in dollars per share)		$ 6.75
Options Exercisable, Weighted-average Remaining Contractual Term (Year)		6 years 36 days
Options Exercisable, Aggregate Intrinsic Value
Options Vested or Expected to Vest (in shares)	[1]	5,779,726
Options Vested or Expected to Vest, Weighted Average Exercise Price (in dollars per share)	[1]	$ 4.62
Options Vested or Expected to Vest, Weighted-average Remaining Contractual Term (Year)	[1]	7 years 146 days
Options Vested or Expected to Vest, Aggregate Intrinsic Value	[1]

[1] Represents the number of vested options at March 31, 2019 plus the number of unvested options expected to vest based on the unvested options outstanding at March 31, 2019