Note 7 - Equity Incentive Plans - Restricted Stock Units Activity (Details) - Restricted Stock Units (RSUs) [Member]	6 Months Ended
	Jun. 30, 2020 $ / shares shares
Outstanding at December 31, 2019 (in shares) | shares	240,229
Outstanding at December 31, 2019, weighted average grant date fair value (in dollars per share) | $ / shares	$ 29.07
Granted (in shares) | shares	53,041
Granted, weighted average grant date fair value (in dollars per share) | $ / shares	$ 63.77
Issued upon vesting (in shares) | shares	(97,620)
Issued upon vesting, weighted average grant date fair value (in dollars per share) | $ / shares	$ 24.51
Forfeited (in shares) | shares	(420)
Forfeited, weighted average grant date fair value (in dollars per share) | $ / shares	$ 27.00
Outstanding at June 30, 2020 (in shares) | shares	195,230
Outstanding at June 30, 2020, weighted average grant date fair value (in dollars per share) | $ / shares	$ 40.78