Note 15 - Net Income Per Share and Common Stock Dividends - Basic and Diluted Net Income Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended												12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Net income, basic and diluted	$ 14,658	$ 13,876	$ 13,579	$ 13,404	$ 4,946	$ 11,704	$ 10,602	$ 10,791	$ 10,616	$ 10,467	$ 10,109	$ 9,835	$ 55,517	$ 38,043	$ 41,027
Average shares outstanding, basic (in shares)													22,619	22,532	22,356
Effect of dilutive shares (in shares)													325	451	436
Average shares outstanding including dilutive shares (in shares)													22,944	22,983	22,792
Net income per share, basic (in dollars per share)	$ 0.65	$ 0.61	$ 0.60	$ 0.59	$ 0.22	$ 0.52	$ 0.47	$ 0.48	$ 0.47	$ 0.47	$ 0.45	$ 0.44	$ 2.45	$ 1.69	$ 1.84
Net income per share, diluted (in dollars per share)	$ 0.64	$ 0.60	$ 0.59	$ 0.58	$ 0.22	$ 0.51	$ 0.46	$ 0.47	$ 0.46	$ 0.46	$ 0.45	$ 0.44	$ 2.42	$ 1.66	$ 1.80