Note 6 - Basic and Diluted Earnings Per Common Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Net income	$ 8,303	$ 8,437	$ 8,531	$ 8,193	$ 8,790	$ 8,410	$ 8,357	$ 10,188	$ 33,464	$ 35,745	$ 29,834
Basic earnings per common share - weighted-average shares (in shares)									32,506	33,477	33,381
Effect of dilutive stock options (in shares)									171	238	297
Diluted earnings per common share - weighted-average shares and assumed conversions (in shares)									32,677	33,715	33,678
Basic earnings per common share (in dollars per share)	$ 0.25	$ 0.26	$ 0.26	$ 0.25	$ 0.26	$ 0.25	$ 0.25	$ 0.30	$ 1.03	$ 1.07	$ 0.89
Diluted earnings per common share (in dollars per share)	$ 0.25	$ 0.26	$ 0.26	$ 0.25	$ 0.26	$ 0.25	$ 0.25	$ 0.30	$ 1.02	$ 1.06	$ 0.89