Note 10 - Warrants - Summary of Warrant Activity (Details) - $ / shares	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Outstanding, Number (in shares)	4,080,005	6,688,849
Outstanding, Weighted average exercise price (in dollars per share)	$ 4.35	$ 4.50
Exercised, Number (in shares)	(121,000)	(2,608,844)
Exercised, Weighted average exercise price (in dollars per share)	$ 4.75	$ 4.75
Outstanding, Number (in shares)	3,959,005	4,080,005
Outstanding, Weighted average exercise price (in dollars per share)	$ 4.33	$ 4.35