Note 10 - Stock Option Plans - Stock Option Activity (Details) - $ / shares	12 Months Ended		204 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2019
Options outstanding, beginning of year (in shares)	207,500	243,500
Options cancelled (in shares)	(16,000)	(8,000)	(282,680)
Options exercised (in shares)	(107,000)	(40,000)	(947,320)
Options granted (in shares)	42,000	12,000	1,356,500
Options outstanding, end of year (in shares)	126,500	207,500	126,500
Options available for grant (in shares)	763,000	805,000	763,000
Options exercisable (in shares)	84,500	179,000	84,500
Granted (in dollars per share)	$ 25.45	$ 7.80
Exercised (in dollars per share)	1.97	2.79
Outstanding (in dollars per share)	8.94	2.33	$ 8.94
Exercisable (in dollars per share)	2.21	1.82	2.21
Minimum [Member]
Granted (in dollars per share)	19.99
Exercised (in dollars per share)	0.69	0.69
Outstanding (in dollars per share)	1.52	0.69	1.52
Maximum [Member]
Granted (in dollars per share)	39.11
Exercised (in dollars per share)	7.80	3.89
Outstanding (in dollars per share)	$ 39.11	$ 7.80	$ 39.11