Note 6 - Stock-based Compensation - Stock Option Activity for Employees and Non-employees (Details) - USD ($) $ / shares in Units, $ in Thousands		6 Months Ended	12 Months Ended
		Jun. 30, 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 Exercise Price, Ending Balance (in dollars per share)		$ 7.03	$ 8.55
Options Outstanding, Weighted-average Remaining Contractual Term, Beginning Balance (Year)		7 years 109 days	7 years 36 days
Options Outstanding, Aggregate Intrinsic Value, Beginning Balance		$ 45	$ 124
Options Granted (in shares)		706,004
Options Granted, Weighted Average Exercise Price (in dollars per share)		$ 2
Options Exercised (in shares)		(74,001)
Options Exercised, Weighted Average Exercise Price (in dollars per share)		$ 1.45
Options Forfeited (in shares)		(14,687)
Options Exercisable (in shares)		1,388,426
Options Exercisable, Weighted Average Exercise Price (in dollars per share)		$ 7.46
Options Exercisable, Weighted-average Remaining Contractual Term (Year)		5 years 328 days
Options Exercisable, Aggregate Intrinsic Value		$ 34
Options Vested or Expected to Vest (in shares)	[1]	2,783,570
Options Vested or Expected to Vest, Weighted Average Exercise Price (in dollars per share)	[1]	$ 7.03
Options Forfeited, Weighted Average Exercise Price (in dollars per share)		$ 16.92
Options Outstanding, Ending Balance (in shares)		2,783,570	2,166,254
Options Vested or Expected to Vest, Weighted-average Remaining Contractual Term (Year)	[1]	7 years 109 days
Options Vested or Expected to Vest, Aggregate Intrinsic Value	[1]	$ 45

[1] Represents the number of vested options at June 30, 2017 plus the number of unvested options expected to vest based on the unvested options outstanding at June 30, 2017.