Note 11 - Stockholders' Equity and Earnings Per Share - Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Net income	$ 67,377	$ 72,835	$ 72,244	$ 66,679	$ 64,648	$ 69,757	$ 73,664	$ 63,816	$ 279,135	$ 271,885	$ 176,042
Basic EPS, income									$ 279,135	$ 271,885	$ 176,042
Basic average common shares outstanding (in shares)									79,999,703	81,131,269	80,262,782
Basic (in dollars per share)	$ 0.85	$ 0.91	$ 0.90	$ 0.83	$ 0.80	$ 0.86	$ 0.91	$ 0.79	$ 3.49	$ 3.35	$ 2.19
Effect of dilutive stock options (in shares)									248,190	476,077	741,768
Diluted EPS, income									$ 279,135	$ 271,885	$ 176,042
Diluted average common shares outstanding (in shares)									80,247,893	81,607,346	81,004,550
Diluted (in dollars per share)	$ 0.84	$ 0.91	$ 0.90	$ 0.83	$ 0.80	$ 0.85	$ 0.90	$ 0.78	$ 3.48	$ 3.33	$ 2.17