Note 15 - Net Income Per Share - 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, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	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, 2019	Dec. 31, 2018	Dec. 31, 2017
Net income	$ 16,649	$ 17,234	$ 16,543	$ 15,641	$ 14,658	$ 13,876	$ 13,579	$ 13,404	$ 4,946	$ 11,704	$ 10,602	$ 10,791	$ 66,067	$ 55,517	$ 38,043
Weighted average shares outstanding - basic (in shares)													22,598	22,619	22,532
Dilutive securities (in shares)													267	325	451
Weighted average shares outstanding - diluted (in shares)													22,865	22,944	22,983
Net income per share, basic (in dollars per share)	$ 0.74	$ 0.76	$ 0.73	$ 0.69	$ 0.65	$ 0.61	$ 0.60	$ 0.59	$ 0.22	$ 0.52	$ 0.47	$ 0.48	$ 2.92	$ 2.45	$ 1.69
Net income per share, diluted (in dollars per share)	$ 0.73	$ 0.76	$ 0.72	$ 0.68	$ 0.64	$ 0.60	$ 0.59	$ 0.58	$ 0.22	$ 0.51	$ 0.46	$ 0.47	$ 2.89	$ 2.42	$ 1.66