Note 6 - Warrants - Summary of Warrant Activity (Details) - $ / shares shares in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Outstanding, Number (in shares)	6,689	7,603
Outstanding, Weighted average exercise price (in dollars per share)	$ 4.50	$ 4.46
Exercised, Number (in shares)	(2,609)	(914)
Exercised, Weighted average exercise price (in dollars per share)	$ 4.75	$ 4.18
Outstanding, Number (in shares)	4,080	6,689
Outstanding, Weighted average exercise price (in dollars per share)	$ 4.35	$ 4.50