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, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Net income	$ 25,940	$ 49,746	$ 51,415	$ 48,941	$ 48,015	$ 46,090	$ 34,831	$ 46,163	$ 176,042	$ 175,099	$ 161,109
Basic EPS, income									$ 176,042	$ 175,099	$ 161,109
Basic average common shares outstanding (in shares)									80,262,782	79,153,762	80,563,577
Basic (in dollars per share)	$ 0.32	$ 0.62	$ 0.64	$ 0.61	$ 0.61	$ 0.58	$ 0.44	$ 0.58	$ 2.19	$ 2.21	$ 2
Effect of dilutive stock options (in shares)									741,768	775,500	731,219
Diluted EPS, income									$ 176,042	$ 175,099	$ 161,109
Diluted average common shares outstanding (in shares)									81,004,550	79,929,262	81,294,796
Diluted (in dollars per share)	$ 0.32	$ 0.61	$ 0.64	$ 0.61	$ 0.60	$ 0.58	$ 0.44	$ 0.57	$ 2.17	$ 2.19	$ 1.98