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, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Net income													$ 41,027	$ 37,187	$ 34,822
Average shares outstanding, basic (in shares)													22,356	22,088	21,839
Effect of dilutive securities (in shares)													436	371	305
Average shares outstanding including dilutive securities (in shares)													22,792	22,459	22,144
Net income per share, basic (in dollars per share)	$ 0.47	$ 0.47	$ 0.45	$ 0.44	$ 0.43	$ 0.42	$ 0.41	$ 0.42	$ 0.40	$ 0.45	$ 0.37	$ 0.37	$ 1.84	$ 1.68	$ 1.59
Net income per share, diluted (in dollars per share)	$ 0.46	$ 0.46	$ 0.45	$ 0.44	$ 0.43	$ 0.41	$ 0.40	$ 0.42	$ 0.39	$ 0.45	$ 0.37	$ 0.37	$ 1.80	$ 1.65	$ 1.57