Note 8 - Stock-based Compensation - Stock Option Activity (Details)	3 Months Ended
	Mar. 31, 2019 USD ($) $ / shares shares
Stock Option Plan 2001 [Member]
Options outstanding (in shares) | shares	6,193
Options outstanding, weighted average exercise price (in dollars per share) | $ / shares	$ 2.95
Options exercised (in shares) | shares	(6,193)
Options exercised, weighted average exercise price (in dollars per share) | $ / shares	$ 2.95
Options outstanding (in shares) | shares
Options outstanding, weighted average exercise price (in dollars per share) | $ / shares
Stock Option Plan 2013 [Member]
Options outstanding (in shares) | shares	196,500
Options outstanding, weighted average exercise price (in dollars per share) | $ / shares	$ 13.84
Options exercised (in shares) | shares	(7,200)
Options exercised, weighted average exercise price (in dollars per share) | $ / shares	$ 7.94
Options outstanding (in shares) | shares	189,300
Options outstanding, weighted average exercise price (in dollars per share) | $ / shares	$ 14.07
Options outstanding, weighted average remaining contractual term (Year)	5 years 73 days
Options outstanding, intrinsic value | $	$ 1,766,000
Options exercisable (in shares) | shares	113,175
Options exercisable, weighted average exercise price (in dollars per share) | $ / shares	$ 10.27
Options exercisable, weighted average remaining contractual term (Year)	4 years 182 days
Options exercisable, intrinsic value | $	$ 1,446,000
Options expected to vest (in shares) | shares	67,569
Options expected to vest, weighted average exercise price (in dollars per share) | $ / shares	$ 19.71
Options expected to vest, weighted average remaining contractual term (Year)	6 years 109 days
Options expected to vest, intrinsic value | $	$ 320,000