Note 15 - Stock-based Compensation - Stock Option Plans (Details) - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Outstanding, shares (in shares)	587,961	623,176	661,771
Outstanding, Weighted average exercise price (in dollars per share)	$ 14.83	$ 13.88	$ 13.89
Granted, shares (in shares)	43,250	76,749	73,403
Granted, Weighted average exercise price (in dollars per share)	$ 43.86	$ 22.92	$ 17.63
Exercised, shares (in shares)	(114,100)	(111,423)	(79,638)
Exercised, Weighted average exercise price (in dollars per share)	$ 15.12	$ 14.97	$ 14.70
Forfeited, shares (in shares)	(3,557)	(541)	(32,360)
Forfeited, Weighted average exercise price (in dollars per share)	$ 26.74	$ 18.36	$ 20.69
Outstanding, shares (in shares)	513,554	587,961	623,176
Outstanding, Weighted average exercise price (in dollars per share)	$ 17.13	$ 14.83	$ 13.88
Exercisable, , shares (in shares)	354,269	385,372	405,832
Weighted-average grant date fair value (in dollars per share)	$ 14.75	$ 7.31	$ 5.11