Note 12 - Stock-based Compensation - Summary of Option Activity Under the Plans (Details) - $ / shares shares in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Outstanding, shares (in shares)	6,264
Outstanding, Weighted Average Exercise Price (in dollars per share)	$ 6.36	$ 6.79
Outstanding, Weighted Average Remaining Contractual Life (Year)	6 years 193 days	6 years 7 days
Granted, shares (in shares)	1,373
Granted, Weighted Average Exercise Price (in dollars per share)	$ 4.52
Cancelled, shares (in shares)	(105)
Cancelled, Weighted Average Exercise Price (in dollars per share)	$ 7.02
Expired, shares (in shares)	(28)
Expired, Weighted Average Exercise Price (in dollars per share)	$ 7.79
Outstanding, shares (in shares)	7,504	6,264
Exercisable, shares (in shares)	4,860
Exercisable, Weighted Average Exercise Price (in dollars per share)	$ 6.58
Exercisable, Weighted Average Remaining Contractual Life (Year)	5 years 18 days
Vested and expected to vest, shares (in shares)	6,749
Vested and expected to vest, Weighted Average Exercise Price (in dollars per share)	$ 6.44
Vested and expected to vest, Weighted Average Remaining Contractual Life (Year)	6 years 76 days