Note 6 - Share-based Compensation - Stock Option Transactions (Details) $ / shares in Units, $ in Thousands	6 Months Ended		12 Months Ended
	Jun. 30, 2019 USD ($) $ / shares shares		Dec. 31, 2018 USD ($) $ / shares shares
Outstanding, January 1, 2019 (in shares) | shares	676,846
Outstanding, weighted average exercise price (in dollars per share) | $ / shares	$ 15.70
Outstanding, weighted average remaining life (Year)	3 years 102 days		2 years 361 days
Outstanding, aggregate intrinsic value | $	$ 1,243		$ 2,230
Granted (in shares) | shares	142,009	[1]
Granted, weighted average exercise price (in dollars per share) | $ / shares	$ 17.68	[1]
Exercised (in shares) | shares	(75,144)
Exercised, weighted average exercise price (in dollars per share) | $ / shares	$ 6.70
Cancelled (in shares) | shares	(54,148)
Cancelled, weighted average exercise price (in dollars per share) | $ / shares	$ 19.36
Outstanding, June 30, 2019 (in shares) | shares	689,563		676,846
Outstanding, weighted average exercise price (in dollars per share) | $ / shares	$ 16.80		$ 15.70
Options exercisable, June 30, 2019 (in shares) | shares	517,593
Options exercisable, weighted average exercise price (in dollars per share) | $ / shares	$ 16.42
Options exercisable, weighted average remaining life (Year)	2 years 273 days
Options exercisable, aggregate intrinsic value | $	$ 1,239

[1] The weighted average grant date fair value of stock options granted was $3.97 per share.