Note 11 - Stock-based Compensation - Summary of Option Activity Under the Plans (Details) - $ / shares	6 Months Ended	12 Months Ended
	Jun. 30, 2018	Dec. 31, 2017
Outstanding, shares (in shares)	5,535,000
Outstanding, Weighted Average Exercise Price (in dollars per share)	$ 7.25
Outstanding, Weighted Average Remaining Contractual Life (Year)	6 years 124 days	6 years 14 days
Granted, shares (in shares)	1,511,000
Granted, Weighted Average Exercise Price (in dollars per share)	$ 6.91
Exercised, shares (in shares)	0
Cancelled, shares (in shares)	(270,000)
Cancelled, Weighted Average Exercise Price (in dollars per share)	$ 7.77
Expired, shares (in shares)	(3,000)
Expired, Weighted Average Exercise Price (in dollars per share)	$ 18.03
Outstanding, shares (in shares)	6,773,000	5,535,000
Outstanding, Weighted Average Exercise Price (in dollars per share)	$ 7.15	$ 7.25
Exercisable, shares (in shares)	4,546,000
Exercisable, Weighted Average Exercise Price (in dollars per share)	$ 6.96
Exercisable, Weighted Average Remaining Contractual Life (Year)	5 years
Vested and expected to vest, shares (in shares)	6,250,000
Vested and expected to vest, Weighted Average Exercise Price (in dollars per share)	$ 7.12
Vested and expected to vest, Weighted Average Remaining Contractual Life (Year)	6 years 32 days