Note 11 - Stock-based Compensation - Summary of Option Activity Under the Plans (Details) - $ / shares	3 Months Ended	12 Months Ended
	Mar. 31, 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 208 days	6 years 14 days
Granted, shares (in shares)	1,313,000
Granted, Weighted Average Exercise Price (in dollars per share)	$ 6.61
Exercised, shares (in shares)	0
Cancelled, shares (in shares)	(24,000)
Cancelled, Weighted Average Exercise Price (in dollars per share)	$ 8.78
Expired, shares (in shares)	(3,000)
Expired, Weighted Average Exercise Price (in dollars per share)	$ 18.03
Outstanding, shares (in shares)	6,821,000	5,535,000
Outstanding, Weighted Average Exercise Price (in dollars per share)	$ 7.12	$ 7.25
Exercisable, shares (in shares)	4,349,000
Exercisable, Weighted Average Exercise Price (in dollars per share)	$ 6.88
Exercisable, Weighted Average Remaining Contractual Life (Year)	5 years 14 days
Vested and expected to vest, shares (in shares)	6,181,000
Vested and expected to vest, Weighted Average Exercise Price (in dollars per share)	$ 7.08
Vested and expected to vest, Weighted Average Remaining Contractual Life (Year)	6 years 102 days