Note 6 - Stock-based Compensation - Stock Option Activities (Details) - $ / shares	3 Months Ended		12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2017
Outstanding (in shares)	164,410	207,108	207,108
Outstanding, weighted average exercise price (in dollars per share)	$ 14.69	$ 14.18	$ 14.18
Outstanding, average remaining contractual term (Year)	5 years 219 days	5 years 328 days	5 years 292 days	6 years 73 days
Granted (in shares)	1,000	5,000
Granted, weighted average exercise price (in dollars per share)	$ 14.63	$ 15.48
Exercised (in shares)
Exercised, weighted average exercise price (in dollars per share)
Forfeited or expired (in shares)		(1,002)
Forfeited or expired, weighted average exercise price (in dollars per share)		$ 14.90
Outstanding (in shares)	165,410	211,106	164,410	207,108
Outstanding, weighted average exercise price (in dollars per share)	$ 14.69	$ 14.28	$ 14.69	$ 14.18
Exercisable (in shares)	38,690	28,456
Exercisable, weighted average exercise price (in dollars per share)	$ 13.93	$ 9.65
Exercisable, average remaining contractual term (Year)	4 years 255 days	1 year 109 days