Note 8 - Warrants - Summary of Warrant Activity (Details) - $ / shares	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Outstanding, number of shares (in shares)	3,959,005	4,080,005
Outstanding, weighted average exercise price (in dollars per share)	$ 4.33	$ 4.35
Exercised, number of shares (in shares)	(3,879,905)	(121,000)
Exercised, weighted average exercise price (in dollars per share)	$ 4.33	$ 4.75
Outstanding, number of shares (in shares)	79,100	3,959,005
Outstanding, weighted average exercise price (in dollars per share)	$ 4.75	$ 4.33