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, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Net income	$ 64,648	$ 69,757	$ 73,664	$ 63,816	$ 25,940	$ 49,746	$ 51,415	$ 48,941	$ 271,885	$ 176,042	$ 175,099
Basic EPS, income									$ 271,885	$ 176,042	$ 175,099
Basic average common shares outstanding (in shares)									81,131,269	80,262,782	79,153,762
Basic (in dollars per share)	$ 0.80	$ 0.86	$ 0.91	$ 0.79	$ 0.32	$ 0.62	$ 0.64	$ 0.61	$ 3.35	$ 2.19	$ 2.21
Effect of dilutive stock options (in shares)									476,077	741,768	775,500
Diluted EPS, income									$ 271,885	$ 176,042	$ 175,099
Diluted average common shares outstanding (in shares)									81,607,346	81,004,550	79,929,262
Diluted (in dollars per share)	$ 0.80	$ 0.85	$ 0.90	$ 0.78	$ 0.32	$ 0.61	$ 0.64	$ 0.61	$ 3.33	$ 2.17	$ 2.19