Note 11 - Net Income Per Share - Basic and Diluted Net Income Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Net income	$ 13,404	$ 10,791
Average shares outstanding (in shares)	22,577	22,492
Dilutive securities (in shares)	365	510
Average shares outstanding including dilutive securities including dilutive securities (in shares)	22,942	23,002
Net income per share, basic (in dollars per share)	$ 0.59	$ 0.48
Net income per share, diluted (in dollars per share)	$ 0.58	$ 0.47