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, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	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, 2017	Dec. 31, 2016	Dec. 31, 2015
Net income, basic and diluted													$ 38,043	$ 41,027	$ 37,187
Average shares outstanding, basic (in shares)													22,532	22,356	22,088
Effect of dilutive securities (in shares)													451	436	371
Average shares outstanding including dilutive securities (in shares)													22,983	22,792	22,459
Net income per share, basic (in dollars per share)	$ 0.22	$ 0.52	$ 0.47	$ 0.48	$ 0.47	$ 0.47	$ 0.45	$ 0.44	$ 0.43	$ 0.42	$ 0.41	$ 0.42	$ 1.69	$ 1.84	$ 1.68
Net income per share, diluted (in dollars per share)	$ 0.22	$ 0.51	$ 0.46	$ 0.47	$ 0.46	$ 0.46	$ 0.45	$ 0.44	$ 0.43	$ 0.41	$ 0.40	$ 0.42	$ 1.66	$ 1.80	$ 1.65