Note 10 - Stockholders' Equity - Summary of RSU Activity (Details) - Restricted Stock Units (RSUs) [Member] - $ / shares	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Outstanding (in shares)	147,015	188,681	217,895
Outstanding, weighted average grant date fair value (in dollars per share)	$ 30.24	$ 26.14	$ 21.21
Granted (in shares)	48,298	46,146	56,567
Granted, weighted average grant date fair value (in dollars per share)	$ 48.65	$ 36.86	$ 35.41
Vested (in shares)	(51,414)	(50,669)	(61,058)	[1]
Vested, weighted average grant date fair value (in dollars per share)	$ 26.32	$ 22.76	$ 18.46	[1]
Canceled (in shares)	(10,068)	(37,143)	(24,723)
Canceled, weighted average grant date fair value (in dollars per share)	$ 30.70	$ 27.88	$ 22.19
Outstanding (in shares)	133,831	147,015	188,681
Outstanding, weighted average grant date fair value (in dollars per share)	$ 38.26	$ 30.24	$ 26.14

[1] The number of RSUs vested includes the shares that we withheld on behalf of employees to satisfy minimum statutory tax withholding requirements. The fair values of the RSUs that vested during 2020, 2019 and 2018 were $1.8 million, $2.1 million, and $1.9 million, respectively.