Note 8 - Stock-based Compensation - Number of Stock Option Awards Exercisable and their Weighted Average Exercise Price (Details)	12 Months Ended
	Dec. 31, 2020 $ / shares shares
Upper Exercise Price Range (in dollars per share)	$ 18
Outstanding Number (in shares) | shares	29,056
Outstanding Weighted Average Remaining Life (Year)	7 years 6 months
Outstanding Weighted Average Exercise Price (in dollars per share)	$ 6.14
Exercisable Number (in shares) | shares	13,036
Exercisable Weighted Average Exercise Price (in dollars per share)	$ 3.27
Exercise Price Range 01 [Member]
Upper Exercise Price Range (in dollars per share)	$ 3
Outstanding Number (in shares) | shares	2,514
Outstanding Weighted Average Remaining Life (Year)	6 years 1 month 6 days
Outstanding Weighted Average Exercise Price (in dollars per share)	$ 2.19
Exercisable Number (in shares) | shares	1,414
Exercisable Weighted Average Exercise Price (in dollars per share)	$ 1.25
Exercise Price Range 02 [Member]
Upper Exercise Price Range (in dollars per share)	$ 6
Outstanding Number (in shares) | shares	12,689
Outstanding Weighted Average Remaining Life (Year)	7 years
Outstanding Weighted Average Exercise Price (in dollars per share)	$ 4.22
Exercisable Number (in shares) | shares	6,854
Exercisable Weighted Average Exercise Price (in dollars per share)	$ 2.48
Exercise Price Range 03 [Member]
Upper Exercise Price Range (in dollars per share)	$ 9
Outstanding Number (in shares) | shares	10,852
Outstanding Weighted Average Remaining Life (Year)	9 years 2 months 12 days
Outstanding Weighted Average Exercise Price (in dollars per share)	$ 7.88
Exercisable Number (in shares) | shares	2,005
Exercisable Weighted Average Exercise Price (in dollars per share)	$ 4.89
Exercise Price Range 04 [Member]
Upper Exercise Price Range (in dollars per share)	$ 12
Outstanding Number (in shares) | shares	2,347
Outstanding Weighted Average Remaining Life (Year)	4 years 6 months
Outstanding Weighted Average Exercise Price (in dollars per share)	$ 10.85
Exercisable Number (in shares) | shares	2,129
Exercisable Weighted Average Exercise Price (in dollars per share)	$ 4.51
Exercise Price Range 05 [Member]
Upper Exercise Price Range (in dollars per share)	$ 15
Outstanding Number (in shares) | shares	559
Outstanding Weighted Average Remaining Life (Year)	4 years 1 month 6 days
Outstanding Weighted Average Exercise Price (in dollars per share)	$ 12.30
Exercisable Number (in shares) | shares	539
Exercisable Weighted Average Exercise Price (in dollars per share)	$ 6.43
Exercise Price Range 06 [Member]
Upper Exercise Price Range (in dollars per share)	$ 18
Outstanding Number (in shares) | shares	95
Outstanding Weighted Average Remaining Life (Year)	4 years 6 months
Outstanding Weighted Average Exercise Price (in dollars per share)	$ 15.39
Exercisable Number (in shares) | shares	95
Exercisable Weighted Average Exercise Price (in dollars per share)	$ 10.57