Note 9 - Warrants - Summary of Warrant Activity (Details) - $ / shares	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Outstanding at beginning of year (in shares)	79,100	3,959,005	4,080,005
Outstanding, weighted average exercise price (in dollars per share)	$ 4.75	$ 4.33	$ 4.35
Exercised, number of shares (in shares)	(79,100)	(3,879,905)	(121,000)
Exercised, weighted average exercise price (in dollars per share)	$ 4.75	$ 4.33	$ 4.75
Outstanding, number of shares (in shares)	0	79,100	3,959,005
Outstanding, weighted average exercise price (in dollars per share)	$ 0	$ 4.75	$ 4.33