Note 12 - Stockholders' Equity and Earnings Per Share - Earnings Per Share (Details) - USD ($) $ / shares in Units, $ 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, 2016	Dec. 31, 2015	Dec. 31, 2014
Net income	$ 48,015	$ 46,090	$ 34,831	$ 46,163	$ 41,446	$ 38,483	$ 45,200	$ 35,980	$ 175,099	$ 161,109	$ 137,830
Basic EPS, income									$ 175,099	$ 161,109	$ 137,830
Basic average common shares outstanding (in shares)									79,153,762	80,563,577	79,661,571
Basic (in dollars per share)	$ 0.61	$ 0.58	$ 0.44	$ 0.58	$ 0.51	$ 0.47	$ 0.57	$ 0.45	$ 2.21	$ 2	$ 1.73
Effect of dilutive stock options (in shares)									775,500	731,219	445,324
Diluted EPS, income									$ 175,099	$ 161,109	$ 137,830
Diluted average common shares outstanding (in shares)									79,929,262	81,294,796	80,106,895
Diluted (in dollars per share)	$ 0.60	$ 0.58	$ 0.44	$ 0.57	$ 0.51	$ 0.47	$ 0.56	$ 0.45	$ 2.19	$ 1.98	$ 1.72